Evidence - Collateral Fact Rule
R v MacIsaac (Ont CA, 2017)
In this criminal case the Court of Appeal discusses the collateral fact rule, which is the rule that a witness should not be impeached (discredited by later conflicting evidence) on issues that are not material to the case:
 The collateral fact rule does not curtail what is otherwise proper cross-examination of a witness; it potentially limits the manner in which answers given may be subsequently challenged by extrinsic evidence: see Sidney N. Lederman, Alan W. Bryant and Michelle K. Fuerst, The Law of Evidence in Canada, 4th ed. (Toronto: LexisNexis Canada Inc., 2014), at pp. 1195-1201. As is often said, if the questioner asks a question that bears on a collateral issue, he or she is “stuck” with the answer, in the sense of not being permitted to lead extrinsic evidence to contradict it. However, this does not prevent proper questions from being put in the first place: see R. v. Krause, 1986 CanLII 39 (SCC),  2 S.C.R. 466, at pp. 474-475 and R. v. Khanna, 2016 ONCA 39 (CanLII), 127 W.C.B. (2d) 613, at para. 9.
 The collateral fact rule is most often engaged when a cross-examiner attempts to challenge the credibility of a witness. Generally speaking, credibility is considered to be collateral, thereby barring the questioner from adducing extrinsic evidence that bears solely on this issue. However, the rule has developed in a manner that admits of a number of exceptions: See Earl J. Levy, Examination of Witnesses in Criminal Cases, 7th ed. (Toronto: Thomson Reuters, 2016), at pp. 509-511. The exception that might have applied in this case is that medical evidence may be adduced to prove that, by virtue of a mental or physical condition, the witness is incapable of telling or is unlikely to tell the truth. In Toohey v. Metropolitan Police Commissioner,  A.C. 595 (H.L.), at p. 608, Lord Pearce held: “…it must be allowable to call medical evidence of mental illness which makes a witness incapable of giving reliable evidence, whether through the existence of delusions or otherwise.” See also R. v. Dietrich, 1970 CanLII 377 (ON CA),  3 O.R. 725 (C.A.), at pp. 742-744.