The collateral fact rule prohibits calling evidence solely to contradict a witness on a collateral fact. The rule does not impact the scope of cross-examination, but rather limits what contradictory evidence can be called to refute a witness’s answer. The rule seeks to preserve trial efficiency and avoid confusion and distraction by preventing the litigation of issues that have only marginal relevance.
See R. v. C.F., 2017 ONCA 480, at para. 58.
Cross-examination on Prior sexual assault allegation
The Court of Appeal for Ontario has twice refused to permit an accused to cross-examine a complainant on prior sexual assault allegations and then call the alleged perpetrators to contradict the complainant, on the basis that the evidence would offend the collateral fact rule: 
R. v. Riley (1992), 11 O.R. (3d) 151, leave to appeal refused, [1993] 2 S.C.R. x; and R. v. B.(A.R.) (1998), 41 O.R. (3d) 361, aff’d 2000 SCC 30, [2000] 1 S.C.R. 781. 
[The prior sexual assault allegations in those cases were made against third parties and the court was rightly concerned about confusing the process by introducing a litany of marginally relevant issues for the trier of fact to decide]. 
In R. v. C.F., 2017 ONCA 480, however, the Court of Appeal upheld the trial judge’s ruling allowing the accused to i) cross-examine the complainant on a specific sexual allegation that the complainant had previously made against the accused, and ii) to call evidence to rebut that allegation. 
The Court took a functional approach, looking at the mischief to which the collateral fact rule is directed.  

In the Court’s opinion, cross-examination and the calling of contradictory evidence was limited to a specific allegation made against the accused, did not unduly lengthen or complicate the proceedings, did not introduce a sideshow of ancillary issues that had to be resolved, and the complainant was not unfairly surprised by the cross-examination (having testified in respect of the allegation at a previous trial).