R v Ewanchuk, 1999 SCJ 10 (Major):

[56] In Esau, supra, at para. 15, the Court stated that, “before a court should consider honest but mistaken belief or instruct a jury on it there must be some plausible evidence in support so as to give an air of reality to the defence”. See also R. v. Osolin, [1993] 4 S.C.R. 595. All that is required is for the accused to adduce some evidence, or refer to evidence already adduced, upon which a properly instructed trier of fact could form a reasonable doubt as to his mens rea: see Osolin, supra, at pp. 653-54, and p. 687.