In similar fact evidence cases, at the admissibility stage, the trial judge’s main task is to weigh the probative value of the evidence against its potential prejudicial effect. The possibility of collusion may significantly affect this balancing.
See R. v. Wilkinson, 2017 ONCA 756 (CanLII), at para. 29.
The theory of similar fact evidence turns largely on the improbability of coincidence. Collusion, by offering an alternative explanation for the “coincidence” of evidence emanating from different witnesses, destroys its probative value, and therefore the basis for its admissibility.
R. v. Shearing2002 SCC 58 (CanLII),Shearing, at para. 40.
Accused to Establish an Air of Reality to the Allegation of Collusion
The dividing line between cases in which collusion is a live issue, and cases in which it is not, is the presence of an “air of reality.” As Binnie J. held in Handy,2002 SCC 56 (CanLII), at para. 111: “The issue is concoction or collaboration, not contact. If the evidence amounts to no more than opportunity, it will usually best be left to the jury.”
Crown must then disprove collusion on balance
 Although collusion is a feature of probative value, it is singled out for special consideration at the admissibility stage. The Crown must disprove the possibility of collusion. Where, there is some evidence of actual collusion, or at least an “air of reality” to the allegations, the Crown is required to satisfy the trial judge, on a balance of probabilities, that the evidence of similar facts is not tainted with collusion. That much would gain admission. 
Handy, at para. 112.
If this threshold test is passed, the jury must determine for itself what weight, if any, to assign to the similar fact evidence.
Shearing, at para. 42.
Inadvertent Collusion
Collusion may be deliberate or inadvertent.  Irrespective, the Crown is held to the same standard of disproving that the evidence was tainted by collusion.
Wilkinson, at para. 39, 45.
Inadvertent or unintentional collusion may occur in a variety of ways, for instance:

  • through the witness viewing media reports: R. v. Dorsey, 2012 ONCA 185 (CanLII), 288 C.C.C. (3d), at para. 29;
  • as a result of 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: R. v. F. (J.) (2003), 2003 CanLII 52166 (ON CA), at para. 77; R. v. B. (C.) (2003), 2003 CanLII 32894 (ON CA), at paras. 40-41.