What is the current state of the law on mistake of fact?

The leading case on mistake of fact is Pappajohn v. The Queen, 1980 CanLII 13 (SCC), [1980] 2 S.C.R. 120. Dickson J. (as he then was) offered the following: Mistake is a defence…where it prevents an accused from having the mens rea which the law requires for the very crime with which he is charged. Mistake of fact is more accurately seen as a negation of guilty intention than as the affirmation of a positive defence. It avails an accused who acts innocently, pursuant to a flawed perception of the facts, and nonetheless commits the actus reus of the offence. Mistake is a defence though, in the sense that it is raised as an issue by an accused.