Mistake (criminal law)
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A mistake of fact may sometimes mean that, while a person has committed the physical element of an offence, because they were labouring under a mistake of fact, they never formed the required mens rea, and so will escape liability for offences that require mens rea. This is unlike a mistake of law, which is not usually a defense; law enforcement may or may not take for granted that individuals know what the law is.
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 someone operating under a mistake of fact will not generally be liable, 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: 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 has not committed a crime.
There is a complex question as to whether the defense of 'mistake' applies to crimes that do not specify a mental element - such as strict liability offences and manslaughter by criminal negligence. Under Australian law, the High Court has specifically ruled against any defense of 'reasonable mistake of fact' in manslaughter cases: The Queen v Lavender (2005) 222 CLR 67. However, the defense of mistake is available to offences of strict liability such as drunk driving: see DPP v Bone  NSWSC 1239. And it is the very availability of the defense of 'mistake' that distinguishes between offences of strict and absolute liability. Mistake of fact is unavailable in respect to absolute liability offences.
Honestly held but unreasonable beliefs
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.