Following Pappajohn v. R., where there is no statutory wording to the contrary:
When an offence requires subjective mens rea, the mistake need merely be honest and reasonable relevant to an assessment of the credibility of the accused.
Where the fault element requires objective negligence, the mistake must be both honest and reasonable.
Where the defence of due diligence is available, the mistake must be both honest and reasonable, with the burden of proof on the accused on regulatory offences.
Where the offence is one of absolute liability, mistake of fact is no defence.
The fourth situation may raise a constitutional challenge.
R. v. Hess; R. v. Nguyen  2 S.C.R. 906
Hess and Nguyen were charged with having sex with a female person under 14 under then s. 146(1) of the Criminal Code (since repealed). In case of H, the judge quashed the indictment on the ground that s. 146(1) violated s. 15 of the Charter. Ont. C.A. reversed this and ordered a new trial. In case of N, Manitoba trial judge convicted and this was upheld by Man. C.A., finding that although impugned provision violated s. 7 (not 15), it was saved by s. 1.
Does s. 146(1) violate s. 7 of the Charter? Can s. 1 save it?
Yes, it violates s. 7 and no, s. 1 can’t save it (It does not violate s. 15). Appeal allowed.
By disallowing the defence of mistake of fact, the impugned provision is one of absolute liability and would entail convicting a mentally innocent man. This cannot be justified under s.1 (rational connection, degree of impairment, proportionality between effect of limit and objective).
McLachlin J. and Gonthier J. dissented.
R. v. Ladue  45 C.R. 287 (Y.T.C.A.)
Ladue had sex with a corpse contrary to what’s now s.182(b) of the Crimianl Code. He appealed the conviction to find out whether the trial judge was right in denying him the defence that he did not know the woman was dead. There was evidence that Ladue was so intoxicated he did not realize the woman was dead.
Can the appellant defend himself as innocent because he was mistaken as to whether the woman was alive or dead?
No. Appeal dismissed.
An intention to commit a crime (in this case, rape), although not the precise crime charged, will provide the necessary mens rea under a statute in the form of s. 167(b) because the accused cannot contend that he was acting lawfully or innocently.