Dangers of eye witness testimony

R. v. Hay, 2013 SCC 61:

[40]                          The credibility and weight that should be given to eyewitness testimony is an issue committed to the ultimate trier of fact ― here, the jury; R. v. Mezzo, [1986] 1 S.C.R. 802, at pp. 844-45.  It is well established that where the Crown relies on an eyewitness identification, the trial judge has a duty to caution the jury regarding the well-recognized frailties of identification evidence; see Mezzo, at p. 845, citing R. v. Turnbull, [1976] 3 All E.R. 549 (C.A.); R. v. Hibbert, 2002 SCC 39, [2002] 2 S.C.R. 445, at paras. 78-79 (Bastarache J., dissenting, although not on this point); R. v. Canning, [1986] 1 S.C.R. 991.  However, a properly instructed jury may conclude, notwithstanding the frailties of eyewitness identification, that the eyewitness' testimony is reliable and may enter a conviction on those grounds.  This may be so even where the Crown has relied on only a single eyewitness; seeMezzo, at p. 844; R. v. Nikolovski, [1996] 3 S.C.R. 1197, at para. 23.

[41]                          Although the duty to assess the credibility and weight of an eyewitness' evidence sits with the jury and, in some circumstances, the testimony of one eyewitness will support a conviction, the jury should not be permitted to convict on the basis of eyewitness testimony that could not support an inference of guilt beyond a reasonable doubt.  In other words, a jury should not be instructed that it may convict based on eyewitness testimony alone where that testimony, even if believed, would necessarily leave reasonable doubt in the mind of a reasonable juror;  R. v. Arcuri, 2001 SCC 54, [2001] 2 S.C.R. 828, at paras. 21-25; R. v. Reitsma, [1998] 1 S.C.R. 769, rev'g (1997) 97 B.C.A.C. 303; R. v. Zurowski, 2004 SCC 72, [2004] 3 S.C.R. 509; United States of America v. Shephard, [1977] 2 S.C.R. 1067, at p. 1080.  Indeed, where the Crown's case consists solely of eyewitness testimony that would necessarily leave reasonable doubt in the mind of a reasonable juror, the trial judge must direct an acquittal upon a motion for directed verdict (Arcuri, at para. 21).