Mistake (criminal law)
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A mistake of fact may sometimes offer exculpation (as in excuse) by allowing a criminal defendant some relief from liability for having broken the law. This is unlike a mistake of law, which is not usually a defense.
Discussion
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 legis neminem 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 requisite mens rea required to constitute the crime.
For example, if a defendant goes into a supermarket and places eight items in a basket which is presented to the cashier for payment in the usual way. Both honestly believe that all eight items have been scanned, and the defendant pays the sum shown on the bill. A store detective, however, notices that a mistake was made by the cashier so that only seven items were actually priced. This detective arrests the defendant after leaving the store. Since the defendant honestly believes that he has become the owner of goods in a sale transaction, he cannot form the mens rea for theft (which is usually dishonesty) when he physically removes them from the store. Accordingly, he should be acquitted.
However, in instances where mens rea is not a requirement for a crime, it will not matter if the mistake is honest or reasonable. This concerns strict liability crimes.
Honestly held but unreasonable beliefs
In some jurisdictions, the defense of mistake is available only if the mistaken belief was honest AND reasonable. This is predominately in the area of specific intent crimes where the defendant has to "knowingly" commit the act (murder, arson, receipt of stolen goods, etc.).
Where this principle applies, the defendant reasonable belief may be imputed according to the standard of a reasonable person. Therefore, if X and Y are competing store owners and bitter enemies and X chases a robber from his store into Y's store with a bat and yells, "Where is that thief? I am going to get him!" and Y, believing he is acting in self defense, shoots and kills X, even though his belief was honest, it was not reasonable and Y will be guilty of murder.
In Australian federal law, it is not a requirement for the defense that a mistake was reasonably held. However, the fact that a belief is unreasonable may be relevant in determining whether the belief was actually held by the person (Criminal Code (Cth) s 9.1).
The leading Supreme Court of Canada case on the mistaken belief is R. v. Park, in which it was held that even unreasonable beliefs must be left to a jury to consider. The issue in most states is the extent to which the test of belief should be subjective or objective.
In English law, the Sexual Offences Act 2003 has introduced a hybrid test of reasonable belief as to consent. The defendant must now be seen to have taken steps to ascertain clearly whether the "victim" was consenting in all the circumstances. This abolishes the defence of a genuine though unreasonably mistaken belief as to the consent.