Collateral Fact Evidence. R v Sanderson
In R v Sanderson (Ont CA, 2017) the Court of Appeal cites the collateral fact rule as follows:
 Watt’s Manual of Criminal Evidence (Toronto: Carswell, Thomson Canada Limited, 2002), describes the rule as follows at para. 22.03, p. 265:. R v Pilgrim
The collateral facts rule prohibits the introduction of evidence for the sole purpose of contradicting a witness’ testimony concerning a collateral fact. The rule seeks to avoid confusion and proliferation of issues, wasting of time and introduction of evidence of negligible assistance to the trier of fact in determining the real issues of the case. It endeavours to ensure that the sideshow does not take over the circus. In general, matters that relate wholly and exclusively to the credibility of a non-accused witness are collateral, hence beyond the reach of contradictory evidence.
In R v Pilgrim (Ont CA, 2017), without referring to it as such, the court elaborates and applies the 'collateral fact rule' (aka 'collateral evidence rule'), that a witness may not be discredited or prejudiced on facts immaterial to the case:
 It did not end there. In his Reasons for Sentence, the trial judge treated the exposure of the children to danger as “a significant aggravating factor.” This was a dominant theme in his reasons. Although the trial judge was “reluctant to find this conduct to have been abuse of a person under 18 within section 718.2(a)(ii.1) of the Criminal Code,” he nevertheless found “the conduct deplorable in the extreme.”. R v MacIsaac
 This characterization – “conduct deplorable in the extreme” – may well have been fair. This feature of the case was a significant aggravating factor on sentencing. But this only confirms the seriously prejudicial nature of this evidence at trial.
 Relying on R. v. Chambers, 1990 CanLII 47 (SCC),  2 S.C.R. 1293, at pp. 1310-11, Ms. Shaikh argues that this evidence was admissible and that the trial judge properly restricted its use to merely assessing Mr. Pilgrim’s credibility. See also R. v. S.G.G., 1997 CanLII 311 (SCC),  2 S.C.R. 716, at para. 63. However, before evidence of this nature may be used for this limited purpose, it must be “properly admissible as part of the narration of the relevant events”: see R. v. Teresinski (1992), 70 C.C.C. (3d) 268 (Ont. C.A.), at p. 278, leave to appeal refused,  S.C.C.A. No. 147. See also R. v. Hogan (1982), 2 C.C.C. (3d) 557 (Ont. C.A.), at pp. 564-65; R. v. Cameron (1995), 1995 CanLII 1283 (ON CA), 22 O.R. (3d) 65 (C.A.), at pp. 72-73; and R. v. Sheriffe, 2015 ONCA 880 (CanLII), 333 C.C.C. (3d) 330, at para. 70, leave to appeal refused,  S.C.C.A. No. 299.
 The danger posed to the appellants’ children was not part of the “narration of the relevant events.” It was not relevant to any material issue. It was not probative of whether the drugs were possessed for the purpose of trafficking, as opposed to personal use. Moreover, it was improperly admitted, based partly on the Crown’s desire to cross-examine the appellants on this issue. Throughout the rest of the trial, the child endangerment claim developed into a serious allegation against Mr. Pilgrim, one that he was not charged with.
 The Chambers line of cases does not provide a basis for the admission of evidence of discreditable conduct or bad character. It prescribes a specific and limited use for otherwise admissible evidence. A prosecutor’s desire to create fodder for cross-examination is not a legitimate path to admission. Admission of highly prejudicial evidence on this basis may actually have the effect of discouraging an accused person from testifying. Conversely, it may induce an accused person to testify just to explain irrelevant evidence that portrays him or her in an unfavourable light. Either way, it has real potential for unfairness. I cannot say that the admission of this evidence impacted on the decisions of the appellants to testify in this case. However, anticipated cross-examination was irrelevant to the decision to admit the evidence in the first place.
 Even if it had been admissible as part of the narration of events, the potential dangers associated with this discreditable conduct evidence were acute, even in a trial by judge alone: see R. v. Villeda, 2011 ABCA 85 (CanLII), 269 C.C.C. (3d) 394, at para. 18. It was essential that the trial judge clearly articulate the limited use he was permitted to make of this evidence – evaluating Mr. Pilgrim’s credibility – and restrict his assessment of that evidence to its limited use. I accept Ms. Shaikh’s submission that the trial judge did not say he convicted Mr. Pilgrim because of this evidence of bad character. One could hardly imagine any judge making such a statement.
In this criminal case, R v MacIsaac (Ont CA, 2017), 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.