R. v. C.G., 2021 ONCA 809 (Ont. CA), November 16, 2021 at paragraphs 27 to 32:

The trial judge said that collusion was a factor in assessing the defence evidence. While the trial judge did not characterize the collusion as between “advertent collusion” and what is commonly called “inadvertent collusion”, as the Crown submitted before us, a fair reading of his reasons demonstrates that the trial judge acceded to the Crown’s submission that the defence evidence was tainted by “inadvertent collusion”. The difficulty with the trial judge’s conclusion on this point is two-fold. First, it is implicit in his comments that the trial judge accepted that the evidence of the defence witnesses was diminished by this “inadvertent collusion”. Yet the trial judge never explains how he took the “inadvertent collusion” into account in his assessment of the defence evidence, apart from stating that he does not dismiss the defence evidence on this ground alone. Second, in spite of his evident acceptance that tainting had occurred, the trial judge failed to explore how the “inadvertent collusion” altered the evidence of each of the defence witnesses, if at all.

As I have intimated, the term “collusion” has been used to describe two different phenomena. The first is deliberate or “advertent collusion”, that is, where witnesses get together and fashion their evidence in concert in order to appear to be reciting a consistent and reliable story. The other, commonly referred to as “inadvertent collusion”, occurs where one witness discusses the events with another witness with the consequence that the evidence of one or both of them may be altered. Put another way, a witness’ evidence may be “inadvertently” impacted by the fact that they have heard the evidence of other witnesses which “can have the effect, whether consciously or unconsciously, of colouring and tailoring their descriptions of the impugned events”: R. v. B. (C.) (2003), 171 C.C.C. (3d) 159 (Ont. C.A.), at para. 40. This point was made in R. v. F. (J.) (2003), 177 C.C.C. (3d) 1 (Ont. C.A.), where Feldman J.A. said, at para. 77:

The reliability of a witness’s account can be undermined not only by deliberate collusion for the purpose of concocting evidence, but also by the influence of hearing other people’s stories, which can tend to colour one’s interpretation of personal events or reinforce a perception about which one had doubts or concerns.

While the issue of collusion most often arises when a court is considering the admissibility of similar fact evidence, it is an issue that has relevance to the evaluation of a witness’ evidence in general.

As this case illustrates, it is unfortunate that the term “inadvertent collusion” has been coined. As I will explain, “advertent collusion” affects the credibility of evidence. “Inadvertent collusion”, or accidental tainting, does not do so. It affects only the reliability of such evidence. As a result, an entirely different analysis is required in determining the impact that “inadvertent collusion” may have on the evidence in question. Yet the term “inadvertent collusion” obscures this because the term “collusion” connotes conspiracy, which is a credibility concern. It would be better if the term “inadvertent collusion” was avoided and replaced by the term “inadvertent tainting”. Given that the term “inadvertent collusion” was used during the matter before us, I will continue to refer to “inadvertent collusion” but as I say, that is a term that would best be avoided going forward.

I will begin with advertent collusion. It is self-evident that this first form of collusion is particularly problematic. Deliberate collusion among witnesses will inevitably undermine the credibility of the evidence given. No court would be comfortable relying on evidence from witnesses who have gotten together and decided on what they are going to say when questioned under oath, at least not without independent corroborating evidence establishing that their evidence is reliable, notwithstanding the collusion.

The second form of collusion, “inadvertent collusion”, is more difficult. The fact that one witness has heard what another witness will say, or for that matter has even discussed what another person’s recollections were, does not mean that either witness is not telling the truth, or is not giving their independent recollection, or that their evidence has been tainted. Indeed, even where the evidence of one of the parties to the discussion is inadvertently affected by what another person has said, the account of that other person may not change. For example, in this case, it is possible that when, during the joint meeting with the defence lawyer, the appellant’s wife offered reasons why the appellant could not be guilty, the appellant did innocently incorporate those reasons into his testimony, but that does not mean that his wife’s observations were inaccurate or that her testimony had changed as a result of their discussion. Nor does it necessarily follow that the appellant’s testimony became unreliable. He may have recognized from his own knowledge the truth and importance of what his wife had observed. The key point is that, unlike advertent collusion which corrupts the evidence of all participants, where inadvertent collusion has occurred, a close examination is required to determine what impact that innocent sharing of information may have had on the evidence of each of the witnesses who is a party to the exchange…