Most criminal law systems in developed states exclude mistake of law as a defense, because allowing defendants to invoke their own ignorance of the law would breach the public policy represented by the Latin maxim: ignorantia juris non excusat. But mistake of fact is sometimes allowed as valid defense because, although the defendant has committed the actus reus of the offense, the defendant may honestly believe in a set of facts that would prevent him or her from forming the requisit ...

See also:

Mistake criminal law, Mistake criminal law - Discussion, Mistake criminal law - Honestly held but unreasonable beliefs

Read more here: » Mistake criminal law: Encyclopedia II - Mistake criminal law - Discussion