Collateral facts rule

R. v. C.L., 2012 ONCA 752 deals with the collateral fact rule. In general the rule provides that a party may not impeach the credit of their opponent's witness by calling witnesses to contradict that witness on collateral matters. But, if the evidence goes to an issue in dispute, and is not called merely to attack credibility, the evidence is proper. The Court writes:

[12] I agree with the trial judge. In R. v. Aalders, [1993] 2 S.C.R. 482, 103 D.L.R. (4th) 700, Cory J., for the majority, described evidence that does not offend the collateral fact rule to include evidence that may be helpful in resolving an essential issue in the case. At p. 498, he said:

It is true that the Crown cannot split its case to obtain an unfair advantage. Nor should the Crown be able to put in evidence in reply on a purely collateral issue. However, it is fit and proper that reply evidence be called which relates to an integral and essential issue of the case. In such circumstances, it would be wrong to deprive the trier of fact of important evidence relating to an essential element of the case. The course of a trial, particularly a criminal trial, must be based upon rules of fairness so as to ensure the protection of the individual accused. However, the rules should not go so far as to deprive the trier of fact of important evidence, that can be helpful in resolving an essential element of the case.

 [13] In this case the trial judge recognized that the pizza receipt should not be admitted solely to undermine the credibility of the appellant. While its admission did have that result, the evidence also was helpful in resolving an essential issue in the case.