Ed.: Canadians and, certainly, Americans are very familiar with the Miranda warnings. Those warnings were mandated in 1966 by the U.S. Supreme Court in the case of Miranda v. Arizona; namely, “The person in custody must, prior to interrogation, be clearly informed that he or she has the right to remain silent, and that anything the person says may be used against that person in court; the person must be clearly informed that he or she has the right to consult with an attorney and to have that attorney present during questioning, and that, if he or she is indigent, an attorney will be provided at no cost to represent him or her.
Less known, but extremely significant and with some application in other situations and contexts, is the Vetrovec warning mandated by the Supreme Court of Canada in 1982, and recently reviewed by the same Court in the case of R. v. Khela. Below are the edited words of Fish J. for the majority:
Legal systems far separated in time and place have long recognized that it is dangerous to rest a criminal conviction on the testimony of a single witness, or on a single piece of evidence. This concern is at least as old as Deuteronomy. It arises because witnesses can lie deliberately or mislead inadvertently, documents can be forged, and other items of evidence can be tampered with or planted.
[T]he evidence of a single witness is nonetheless sufficient in Canada to support a conviction for any offence other than treason, perjury or procuring a feigned marriage. Many serious crimes might otherwise go unpunished. But where the guilt of the accused is made to rest exclusively or substantially on the testimony of a single witness of doubtful credit or veracity, the danger of a wrongful conviction is particularly acute.
It is therefore of the utmost importance, in a trial by judge and jury, for the jury to understand when and why it is unsafe to find an accused guilty on the unsupported evidence of witnesses who are “unsavoury,” “untrustworthy,” “unreliable,” or “tainted.”…
[A] specific instruction [by the judge to the jury] is sometimes required in this regard not because jurors are thought to be unintelligent, but rather because they might otherwise be uninformed. It is meant to bring home to lay jurors the accumulated wisdom of the law’s experience with unsavoury witnesses.
Without a cautionary instruction, however, jurors may appreciate neither the need nor the reasons for skepticism and particular scrutiny in dealing with witnesses of this sort. Essentially for that reason, trial judges may – and in some cases must – include in their charges “a clear and sharp warning to attract the attention of the juror[s] to the risks of adopting, without more, the evidence of the witness”: (Vetrovec v. The Queen,  1 S.C.R. 811, at p. 831).