 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued November 25, 2014               Decided April 21, 2015

                         No. 13-7104

                     MELISSA STANDLEY,
                        APPELLANT

                               v.

KAREN EDMONDS-LEACH, OFFICER, DISTRICT OF COLUMBIA
LIBRARY POLICE DEPARTMENT AND DISTRICT OF COLUMBIA,
             A MUNICIPAL CORPORATION,
                     APPELLEES


         Appeal from the United States District Court
                 for the District of Columbia
                     (No. 1:11-cv-01770)


    Anitha W. Johnson argued the cause and filed the briefs for
appellant.

     Mary L. Wilson, Senior Assistant Attorney General, Office
of the Attorney General for the District of Columbia, argued the
cause for appellees. With her on the brief were Irvin B. Nathan,
Attorney General, Todd S. Kim, Solicitor General, and Loren L.
AliKhan, Deputy Solicitor General.
                                 2

    Before: ROGERS, KAVANAUGH and PILLARD, Circuit
Judges.

     Rogers, Circuit Judge: Melissa Standley appeals the
judgment on her tort claims for D.C. Public Library Special
Police Officer Karen Edmonds-Leach and the District of
Columbia on the ground that the district court abused its
discretion in allowing the defendants to call a witness they failed
to identify prior to trial in accordance with Federal Rule of Civil
Procedure 26(a). Standley maintains, as she argued in the
district court, that the witness’s testimony was not offered
“solely for impeachment,” as the rule’s exception requires, and
that the error was prejudicial. We agree that the district court
erred as a matter of law in misstating the exception under Rule
26(a). Because the witness’s testimony was not confined to
impeachment and because the outcome of the trial turned on the
jury’s assessment of the credibility of Standley and Officer
Edmonds-Leach, we further agree that the testimony of the
relatively disinterested witness likely influenced the outcome of
the trial. Accordingly, this court cannot say with fair assurance
that the district court’s error did not affect Standley’s substantial
rights, and we must reverse and remand the case for a new trial.

                                 I.

     On February 3, 2011, Standley went to a D.C. public library
to complete her homework and study for upcoming college
exams. She sat in an area of the library reserved for children,
even though she was too old to sit in that section. Officer
Edmonds-Leach (hereinafter “Officer Leach”) asked Standley
to move. Standley then relocated to the young-adult area, even
though she was too old to sit there, too. After the officer again
asked Standley to move, an altercation ensued, and the officer
arrested Standley. Standley sued Officer Leach and the District
of Columbia for the unconstitutional use of excessive force and
                                3

common law torts. At trial, Standley and Officer Leach disputed
the specifics of their encounter at the library. Other than an
inconclusive video, which both Standley and the officer argued
supported their version of the incident, the only other evidence
about the events at the library was provided by Wendell Kellar,
a librarian, whose testimony is at issue in this appeal.


                                 A.
     At trial, Standley testified that around 5 p.m. she went to a
library near her home to study, as she did three to four times a
week; she was a nursing student at the University of the District
of Columbia. The children’s area of the library is restricted to
children under thirteen years of age and adults accompanying
them, and the section for young adults is reserved for those aged
thirteen to nineteen. Although Standley was twenty-one years
old, the adult section was full, so Standley put her things on a
table in the children’s section, where she had sat previously
when the library was crowded. Officer Leach asked Standley to
leave the area for children. Standley moved, first walking
through the adult section to see if there were any available seats
and then, because there were not, as she claimed could be seen
in the video, to the young-adult area. She explained that
although some adult seats were not presently occupied, other
library patrons had left the seats temporarily to get books or to
socialize, so those seats were not available for use.

     Ten or fifteen minutes later, after Standley had opened her
computer and “start[ed] [her] activities,” Officer Leach asked
Standley to leave the area for young adults. Standley asked the
officer to make an exception because the adult section was full.
 Officer Leach said that she did not care, Standley could not sit
in the young-adult area. Standley told the officer that this was
“real petty” because there were no seats available in the adult
section. The officer laughed and said she knew it was. Standley
                               4

also questioned how she could be asked to leave without first
verifying her age; when Officer Leach requested identification,
Standley said she had none with her. Standley stood up and
walked towards the adult section. As she did so, she mumbled
under her breath the word “bitch” once but did not use any other
profane language. Officer Leach “charge[d] after” her, got “in
[her] face” and asked “what did you say, what did you say,”
trying to provoke a confrontation.

     The officer then told Standley to leave the library and
pointed towards the exit. Standley put down her belongings in
the adult section, intending to pack and leave as instructed. She
asked the officer, who was standing so close that she was
touching Standley, to give her “a little space to pack up [her]
belongings.” The officer replied, “I’m not going no mother[-
]fuckin’ where, make me move, make me move.” “Before I
knew it,” Standley said, “[Officer Leach] hit me in the
face, . . . wrapping her arm around my neck and hitting me
repeatedly and then throwing me into the bookshelves.”
According to Standley, the video showed the officer putting her
in a headlock and throwing her into a bookshelf. Standley
claimed the officer threw her down on the ground and put her
knee in her back. Also, whenever Standley tried to lift her head
to prevent her mouth from touching the ground, because she had
asthma and “couldn’t breathe,” the officer slammed Standley’s
head onto the floor. The officer told her to “shut the F up,” and
said she was resisting arrest. The officer handcuffed Standley,
and she was transported to the police station and charged with
unlawful entry and disorderly conduct. At some point, Standley
was also charged with assault on a police officer.

     Officer Leach’s trial testimony depicted the events leading
to Standley’s arrest quite differently. According to the officer,
she saw Standley sitting in the children’s area and asked her to
move to the adult section, in which there were seats fewer than
                                5

five minutes earlier. When Standley objected to moving
because there were no other seats, the officer explained the
library’s policy regarding the age-restricted sections and again
asked Standley to move. Standley left the children’s area.
Later, Officer Leach noticed Standley sitting in the section for
teenagers and returned to tell Standley to move to the adult
section. Standley asserted that she had sat in the area for young
adults before and kept saying there were no other seats. The
officer responded that she could see seats in the adult section.
Standley continued to object and also asked, “how do you even
know what my age is?” After Standley told the officer she was
only nineteen, Officer Leach requested her identification. At
that point, Officer Leach testified, Standley “went into a frenzy,”
“started cursing,” and said “I didn’t know you need to bring no
fuckin’ ID to the fuckin’ library.” Standley continued to spout
profanities, and told Officer Leach she was “just being a fuckin’
bitch.”

     Officer Leach told Standley to leave the library. Standley
sat there and repeated that the officer was “just a bitch.”
Standley began to pack her things, and then with her laptop in
hand walked towards the adult section. Officer Leach followed
Standley, calling out “ma’am, ma’am” to get Standley’s
attention. Standley ignored the officer, except to say again
“[y]ou’re just being a bitch.” Standley put her belongings down
on a table, as if she were going to sit down. Officer Leach said,
“ma’am, this is the last and final time that I’m going to tell you
to leave,” and pointed towards the exit. The officer “stood
almost to the side of her,” and Standley stated loudly, “bitch, get
the fuck away from me. Bitch, get out of my face.” Other
library patrons turned around to see what was happening.

     Because Standley refused to leave, Officer Leach proceeded
to arrest her for unlawful entry. The officer grabbed Standley’s
arm, but Standley “snatche[d] away.” She tried to grab
                                6

Standley’s arm again, but Standley once more pulled back.
They continued to struggle for “maybe 5 to 10 minutes,”
eventually “end[ing] up in[] the stacks where the books are,”
and at some point they fell to the floor. Library patrons gathered
around Standley and the officer, as the video showed. While
Standley was on the floor, Officer Leach restrained her, pressing
her hand, not her knee, on Standley’s back. The officer denied
that she punched Standley in the face, placed her in a headlock,
slammed her head against the floor, or used any profanity during
the incident.

                                B.
     All of the criminal charges against Standley were dropped
before trial. Standley filed this suit, alleging excessive force in
violation of the Fourth Amendment and common law claims for
false arrest, false imprisonment, assault, and battery against
Officer Leach and the District of Columbia (collectively “the
defendants”). (Standley also filed a malicious prosecution claim
that was dismissed at the close of all the evidence and has not
been pursued on appeal.)

     Prior to trial, Standley filed a motion in limine to preclude
the defendants, pursuant to Federal Rules of Civil Procedure
26(a) and 37(c), from calling non-party witnesses, including
librarian Wendell Kellar, because the defendants did not
disclose any witnesses during discovery. The defendants agreed
not to call Kellar, among others, as witnesses, and the district
court denied Standley’s motion as moot. The defendants,
however, subsequently moved to reinstate Kellar as a witness,
arguing that Kellar’s testimony would “corroborate Officer
Edmonds-Leach’s testimony that [Standley] was disorderly
while at the library,” and was therefore “crucial to the defense.”
 Standley opposed the motion, emphasizing the defendants knew
about Kellar and his knowledge of the incident but failed to
identify him as a potential witness pretrial. The district court
                                 7

denied the defendants’ motion to reinstate Kellar as a witness
during their case-in-chief. But the district court reserved ruling
on whether Kellar could testify for impeachment purposes,
stating that “the rules are very lax on impeachment witnesses
and whether notice must be given for impeachment witnesses.”
Trial Tr. 9 (May 28, 2013) (morning session).

     After Standley testified at trial, the defendants asked to call
Kellar as an impeachment witness. They argued that Kellar
would impeach Standley’s testimony that during the incident at
the library (1) she did not use profanity, except for muttering an
expletive once, and (2) there were no seats available in the adult
section. Standley objected, countering that “it would be
prejudicial and against [] the basic rules to allow defendants to
suddenly use someone for impeachment that was clearly
discoverable” before trial. The district court reserved ruling
until after the officer testified.

     Following Officer Leach’s testimony, the defendants
renewed their request to call Kellar for the purpose of
impeachment. Standley objected, explaining that Kellar’s
testimony would not be proper impeachment evidence because
it would be used to corroborate Officer Leach’s version of
events, with Kellar “only testifying to a different version than
[Standley].” Standley argued that admission of Kellar’s
testimony would violate the discovery rules and would be
unduly prejudicial to her, noting, for example, that she “would
have prepared her case differently” had she known he would be
called as a witness.

    The district court allowed Kellar to testify as an
impeachment witness. The court found that he was “an
impeaching witness and not called solely for corroboration.”
Trial Tr. 68 (May 29, 2013) (afternoon session). The court
explained that, “at least as his testimony is represented” by the
                                8

defendants, Kellar “will be testifying on at least two areas that
[Officer Leach] has not testified to.” Id. In particular, Kellar
would testify about the number of seats that were available in
the adult area, a subject on which the officer had not “claimed
to . . . give any definitive testimony,” and “as a person who was
seated or standing . . . some distance away, he would be . . . in
the best position to testify as to the volume of the remarks that
[Standley] was making.” Id. The district court, therefore, found
that Kellar’s testimony was “more than corroboration; that in
fact, he has independent impeaching evidence to offer.” Id. at
69. Standley reiterated her objection that the Federal Rules of
Civil Procedure require an undisclosed witness to be offered
solely for impeachment, and here the testimony was offered by
the defendants not only to impeach Standley but also to
corroborate Officer Leach. The district court again rejected that
argument, stating that this is “a classic case of impeachment”:
“[Standley] said one thing, and [Kellar] is going to say
something else. Whether impeachment and rebuttal crisscross
is a matter of semantics.” Trial Tr. 6 (May 30, 2013) (morning
session).

     At trial, Kellar testified about the two areas of impeachment
approved by the district court. First, Kellar testified that during
the incident at the library he heard yelling and cursing from
someone other than Officer Leach, with whose voice he was
familiar. That part of his testimony contradicted, and thus
tended to impeach, Standley’s testimony that she did not raise
her voice and that it was Officer Leach who swore at her.
Second, Kellar testified that at the time of the incident the
library was not particularly crowded, and there was adequate
seating available in the adult section, thereby tending to impeach
Standley’s testimony that she needed to sit in the young-adult
section.     But Kellar also testified on another matter.
Specifically, he testified that he heard Officer Leach instruct
Standley to leave the library. That was not impeachment, as
                                9

Standley had testified to the same effect.

     The jury returned a verdict in the defendants’ favor, and the
district court entered judgment for the defendants. Standley
appeals. Our review of the district court’s decision to admit
Kellar’s testimony is for abuse of discretion. See United States
v. Garner, 396 F.3d 438, 440 (D.C. Cir. 2005).

                               II.

     Federal Rule of Civil Procedure 26(a)(1)(A)(i) requires that
a party “without awaiting a discovery request” make an initial
disclosure to the opposing party that includes “the name and, if
known, the address and telephone number of each individual
likely to have discoverable information – along with the subjects
of that information – that the disclosing party may use to support
its claims or defenses, unless the use would be solely for
impeachment.” “In addition to the disclosures required by Rule
26(a)(1),” a party must provide “the name and, if not previously
provided, the address and telephone number of each witness”
that “it may present at trial other than solely for impeachment.”
Fed. R. Civ. P. 26(a)(3)(A)(i). Pursuant to Rule 37(c)(1), if a
party fails to identify a witness as required by Rule 26(a), the
party cannot use that witness to supply evidence at trial, unless
the failure was substantially justified or is harmless. Fed. R.
Civ. P. 37(c)(1); see Musser v. Gentiva Health Servs., 356 F.3d
751, 758 (7th Cir. 2004). In other words, a party need not
disclose a witness pursuant to Rule 26(a) if the evidence will be
used “solely for impeachment,” and the witness may testify at
trial even if not disclosed beforehand. Fed. R. Civ. P.
26(a)(1)(A)(i), 26(a)(3)(A)(i), 37(c)(1); Advisory Committee
Notes on 1993 Amendments to subdivision (a) of Rule 26,
subdivision (c) of Rule 37.

                               A.
                                  10

     The defendants concede that they did not identify Kellar as
a witness in their pretrial disclosures or discovery responses, and
they do not maintain on appeal that their failure to identify
Kellar before trial was substantially justified or harmless. See
Fed. R. Civ. P. 37(c)(1). Kellar’s testimony, therefore, was
admissible at trial only if it is subject to Rule 26(a)’s “solely for
impeachment” exception. See Advisory Committee Note on
1993 Amendments to subdivision (c) of Rule 37; Hammel v. Eau
Galle Cheese Factory, 407 F.3d 852, 869 (7th Cir. 2005).1

      By contrast to substantive evidence, “which is offered to
establish the truth of a matter to be determined by the trier of
fact,” impeachment evidence is “offered to discredit a witness
to reduce the effectiveness of her testimony by bringing forth
evidence which explains why the jury should not put faith in her
. . . testimony.” Chiasson v. Zapata Gulf Marine Corp., 988
F.2d 513, 517 (5th Cir. 1993) (internal quotation omitted), cert.


        1
            Standley has not argued on appeal that Kellar’s testimony
was inadmissible because the defendants failed to identify him in
response to her properly served discovery requests. Under Federal
Rule of Civil Procedure 26(b)(1), “[p]arties may obtain discovery
regarding any nonprivileged matter that is relevant to any party’s
claim or defense.” By contrast to Rule 26(a), Rule 26(b) does not
on its face protect from disclosure evidence that will be used for
impeachment at trial. See Varga v. Rockwell Int’l Corp., 242 F.3d
693, 697 (6th Cir. 2001); Elion v. Jackson, 544 F. Supp. 2d 1, 6-7
(D.D.C. 2008); see also 8 CHARLES ALAN WRIGHT & ARTHUR R.
MILLER, FED. PRAC. & PROC. CIV. § 2015 (3d ed. 2014) (Although
“[t]he initial disclosure requirements exclude items that the
disclosing party may use ‘solely for impeachment,’ . . . no such
categorical limitation applies to material sought through
discovery . . . . The fact that the party responding to discovery
intends to use the material only for impeachment does not take it
out of the realm of discoverable material if it is otherwise relevant.”
(footnote omitted)).
                                11

denied, 511 U.S. 1029 (1994); see Friedman v. Rehal, 618 F.3d
142, 153-54 (2d Cir. 2010) (citing Chiasson, 988 F.2d at 517).
Impeachment evidence used “[t]o attack the credibility of
witnesses by the presentation of evidence showing that facts
asserted or relied upon in their testimony are false . . .
impeach[es] by contradiction.” Wegener v. Johnson, 527 F.3d
687, 691 (8th Cir. 2008). Impeachment by contradiction is an
established way to impeach a witness’s credibility. See, e.g.,
United States v. Miller, 738 F.3d 361, 376-77 (D.C. Cir. 2013);
United States v. Fonseca, 435 F.3d 369, 375 (D.C. Cir. 2006)
(citing WEINSTEIN’S FEDERAL EVIDENCE §§ 607.06[1],
608.20[3][a] (2d ed. 2005)). Kellar impeached Standley by
contradicting her about whether she yelled and cursed and the
availability of seats for adults at the library, so that testimony
was permissible impeachment evidence.

     But “separate and apart from whether [Kellar] contradicted
[Standley’s] testimony,” Kellar’s testimony also “tended ‘to
establish the truth of a matter to be determined by the trier of
fact.’” Klonoski v. Mahlab, 156 F.3d 255, 270 (1st Cir. 1998)
(quoting Chiasson, 988 F.2d at 517); see also United States v.
Sanchez-Robles, 927 F.2d 1070, 1078 (9th Cir. 1991),
disapproved of on other grounds by United States v. Heredia,
483 F.3d 913 (9th Cir. 2007). In particular, if credited by the
jury, Kellar’s testimony about Standley’s behavior tended to
show Officer Leach had probable cause to arrest Standley for
disorderly conduct. The defendants themselves highlighted this
when they first moved to reinstate Kellar as a witness, arguing
that his “testimony w[ould] corroborate Officer Edmonds-
Leach’s testimony that [Standley] was disorderly while at the
library” and was thus “crucial to the defense.” Also, Kellar
testified that Officer Leach ordered Standley to leave the library.
Standley disputed only when, not whether, she was told to leave,
so Kellar’s testimony on this point served little or no
impeachment-by-contradiction purpose. Cf. Sterkel v. Fruehauf
                                12

Corp., 975 F.2d 528, 532 (8th Cir. 1992); Martin v. United
States, 127 F.2d 865, 866 (D.C. Cir. 1942).

     In applying Rule 26(a)’s impeachment exception, some
courts have concluded that the impeachment exception is limited
to evidence that has no potential utility other than impeachment.
For example, in Chiasson, 988 F.2d at 517-18, an oft-cited
authority, the Fifth Circuit held that a video surveillance tape
served in part a substantive function so “regardless of its
impeachment value,” it should have been disclosed prior to trial.
The First Circuit in Klonoski, 156 F.3d at 270 (citing Chiasson,
988 F.2d at 517-18), and other courts, have taken the same
approach. See Searles v. Van Bebber, 251 F.3d 869, 877 (10th
Cir. 2001); Brooks v. Kerry, 37 F. Supp. 3d 187, 204-05 (D.D.C.
2014); Newsome v. Penske Truck Leasing Corp., 437 F. Supp.
2d 431, 434-36 (D. Md. 2006). Likewise, in Wilson v. AM
General Corp., 167 F.3d 1114, 1122 (7th Cir. 1999), the
Seventh Circuit concluded that because the testimony of
witnesses offered to impeach was a part of the defendant’s
“primary line of defense,” the witnesses should have been
disclosed prior to trial and their testimony was properly
excluded. On the other hand, the Seventh Circuit has also held
that undisclosed evidence with both impeachment and
substantive qualities may be presented at trial so long as it is
strictly used to impeach. DeBiasio v. Ill. Cent. R.R., 52 F.3d
678, 686 (7th Cir. 1995); see Hammel, 407 F.3d at 870 n.13 (in
light of forfeiture, declining to address whether testimony was
used for both impeachment and substantive purposes, “a
scenario which may not be covered by the [solely for
impeachment] exception” (citing Wilson, 167 F.3d at 1122;
Klonoski, 156 F.3d at 270; DeBiasio, 52 F.3d at 686)).
Specifically, that court held that it was error to exclude evidence
the defendant offered not to prove its defense, but rather to
impeach the plaintiff’s expert witness. DeBiasio, 52 F.3d at
686. Under either approach, the courts have focused on the
                                13

word “solely” and our sister circuits have read that term strictly.
See 8A CHARLES ALAN WRIGHT & ARTHUR R. MILLER, FED.
PRAC. & PROC. CIV. § 2053 n.57 (3d ed. 2014) (collecting
cases).

      The district court in Hayes v. Cha, 338 F. Supp. 2d 470,
503-04 (D.N.J. 2004), summarized the competing considerations.
On the one hand, the district court acknowledged that some
circuits, such as the Seventh Circuit in DeBiasio, 52 F.3d at 686,
reject the proposition that “solely” means that the evidence can
have no substantive non-impeachment value. Hayes, 338 F.
Supp. 2d at 503. Those courts reason that evidence used to
attack a witness’s credibility often contains some substantive
element, and reading Rule 26(a)’s “solely for impeachment”
exception to bar use of such evidence if not earlier disclosed
could “result in an erosion of evidence capable of warranting the
impeachment designation.” Id. (quotation omitted). On the
other hand, the district court recognized that such an approach
“strikes at the heart of the amended rules’ broad intent” in favor
of disclosure. Id. “Automatic disclosure was adopted to end two
evils that had threatened civil litigation: expensive and
time-consuming pretrial discovery techniques and
trial-by-ambush.” Id. (citing Advisory Committee Note on 1993
Amendments to subdivision (a) of Rule 26). A too expansive
reading of the impeachment exception “could cause a resurgence
of these evils.” Id.

     “Because a district court by definition abuses its discretion
when it makes an error of law, the abuse-of-discretion standard
includes review to determine that the discretion was not guided
by erroneous legal conclusions.” Koch v. Cox, 489 F.3d 384,
388 (D.C. Cir. 2007) (internal quotations omitted). Here, the
district court rejected Standley’s objection to Kellar’s testimony
as “a matter of semantics.” Trial Tr. 6 (May 30, 2013) (morning
session). The court ruled that Kellar could testify because he
                                 14

was “an impeaching witness and not called solely for
corroboration.” Trial Tr. 68 (May 29, 2013) (afternoon session).
Under Rule 26(a), however, the question is whether the
testimony will be used “solely for impeachment,” not whether it
will be used solely for corroboration. See Fed. R. Civ. P.
26(a)(1)(A)(i), 26(a)(3)(A)(i). By shifting the critical word
“solely” to modify “corroboration” rather than “impeachment,”
the district court applied the wrong test in deciding whether to
admit Kellar’s testimony and thus abused its discretion. See
Koch, 489 F.3d at 388.

     By so proceeding, the district court never addressed the
considerations that courts have found relevant under the several
interpretations of the scope and nature of Rule 26(a)’s
impeachment exception, and the opportunity to explore the
contours of that exception was lost. Would the district court
have ruled Kellar’s testimony was admissible even though he
had not been identified by the defendants who knew his
testimony was important to the substance of their defense?
Would the district court have struck Kellar’s testimony when it
was used by defendants for more than impeachment? We do not
know because the district court’s misstatement of the rule
prematurely ended the court’s analysis. In light of the district
court’s legal error we have no occasion to decide which of the
competing approaches to the “solely for impeachment” exception
should be adopted in this circuit. Under either approach, it is not
clear that Kellar’s testimony would have been admissible
because aspects of his testimony constituted “dual functionality”
evidence, and he testified to at least one substantive point that
had no impeachment value at all.

                                 B.
     The question remains whether the error was harmless. This
court “will reverse an erroneous evidentiary ruling . . . only if the
error affects a party’s substantial rights.” Huthnance v. District
                                 15

of Columbia, 722 F.3d 371, 377 (D.C. Cir. 2013) (citing Fed. R.
Civ. P. 61). “This analysis depends on a number of factors,
including the closeness of the case, the centrality of the issue in
question, and the effectiveness of any steps taken to mitigate the
effects of the error.” Id. at 381.

      As a preliminary matter, the defendants’ position that
Standley forfeited any claim of prejudice lacks merit. In her
opening brief, Standley contends that the district court’s
admission of Kellar’s testimony “was manifestly prejudicial.”
Appellant’s Br. 10. Standley pointed to the district court’s
erroneous evidentiary ruling, and “the circumstances of the case
. . . make clear . . . that the [erroneous] ruling . . . was harmful,”
Shinseki v. Sanders, 556 U.S. 396, 410 (2009), so Standley’s
assertion of prejudice sufficed, and she was not required to make
any further argument on appeal to preserve that issue. See id.
Throughout the district court proceedings Standley acted to
protect her rights under the Federal Rules of Civil Procedure and
to preclude Kellar’s testimony because its admission would be
harmful to her case. Before trial, she sought to discover defense
witnesses, specifically requesting that the defendants “[i]dentify
all individuals with personal knowledge of any facts material to
the issues raised in this litigation.” Pl.’s Mot. in Limine to
Exclude Witnesses and Records Exh. 1, at 1 (Apr. 22, 2013); see
Fed. R. Civ. P. 26(b). Standley also moved in limine to preclude
the defendants from calling any non-party witnesses, based on
their failure of disclosure during discovery. See Pl.’s Mot. in
Limine to Exclude Witnesses and Records (Apr. 22, 2013).
Additionally, she opposed the defendants’ subsequent motion to
redesignate Kellar as a witness at trial. When the defendants
later moved to present Kellar as an impeachment witness,
Standley opposed the request, emphasizing among other things
that allowing him to testify would prejudice her.

    On the merits, we conclude that the district court’s legal
                                16

error was not harmless. The prejudice to Standley goes beyond
hindering her ability to plan for trial in deciding who to call as a
witness and in preparing cross-examination of defense witnesses,
although that typically is what the federal rules governing
discovery are designed to avoid. See Advisory Committee Note
on 1993 Amendments to subdivision (a) of Rule 26; Hayes, 338
F. Supp. 2d at 503. Aside from a largely indeterminate video,
which both parties argued to the jury supported their version of
events, the evidence at trial pitted Standley’s testimony against
that of Officer Leach. Under the circumstances, it is highly
likely that Kellar’s testimony influenced the jury’s assessment of
the evidence. Although employed by the District of Columbia,
Kellar was not involved in the altercation or lawsuit and thus
probably would have been viewed by reasonable jurors as an
impartial witness who corroborated part of the officer’s version
of events. The defendants anticipated as much in their closing
argument, telling the jury that “[b]ecause [it] heard two very
different versions” of the incident, credibility was paramount.
The jury might have credited Kellar’s testimony, in particular,
because, as the defendants argued in closing, he “ha[d] no ax to
grind in this case,” yet he supported Officer Leach’s version of
events, which made her appear credible and her version more
believable. Importantly, too, Kellar testified that he heard
yelling and cursing, from someone other than Officer Leach.
That testimony tended to show that the officer had probable
cause to arrest Standley for disorderly conduct and that the
defendants could not be held liable for false arrest and false
imprisonment. Therefore, there is a significant chance that
Kellar’s testimony influenced the outcome of the case. See
Huthnance, 722 F.3d at 381. The defendants nonetheless suggest
that affirmance is required of at least Standley’s excessive force
claim because Kellar did not testify about the physical encounter
and arrest, but given the “she said/she said” nature of the critical
evidence, it is difficult to imagine how Kellar’s testimony
corroborating Officer Leach would not have affected the jury’s
                                17

view of the parties’ credibility in general, even as to those facts
relevant to excessive force.

      Accordingly, this court cannot say with fair assurance that
the district court’s legal error did not affect Standley’s
substantial rights, and we must reverse and remand for a new
trial.
