A judge or jury may choose not to rely upon a witness’s testimony when counsel demonstrates that the witness has made a contradictory statement before trial; however, counsel must ensure that the record includes proof of the inconsistency.
Proof may be the witness’s acknowledgement that they made a different statement on a prior occasion. However, if a witness denies making the prior statement, counsel must take proper steps to prove what was said before trial (see sections 10 and 11 of the Canada Evidence Act).
By way of example, if there is a recorded statement to the police, counsel will obtain a transcript and confront the witness with the relevant portion of their statement at trial. If the witness acknowledges that the transcript accurately reflects what they said at the time, counsel may draw upon the inconsistency during final submissions.
If the witness denies having uttered the statement in the transcript, counsel must play the recorded statement to prove the inconsistency. Otherwise, it cannot be used to undermine the testimony of the witness.
As stated by Justice Beveridge, speaking for a unanimous Court of Appeal in R. v. Mauger,  N.S.J. No. 193, beginning at para. 25:
While one might rightly be suspicious that the transcript was indeed an accurate reflection of what Mr. Morrison told the police, Morrison plainly denied that he had said those words and challenged defence counsel to play the recording. Unfortunately, counsel did not.
If he had, it would have been plain that either Mr. Morrison had, in fact, related different details to the police or the transcript was not accurate. Like the trial judge, we simply do not know.
Justice Beveridge continues at para. 30:
…the record plainly demonstrates that no prior inconsistent statement was established before the trial judge.