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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 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.
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.
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 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.
Mistake of fact may be a defence in criminal law if is genuine, whether or not it is reasonable.
In DPP v Morgan an RAF officer told three other officers to have sex with his wife, and that she would pretend to refuse just to be stimulating. They pleaded mistake, and the jury did not believe them. The House of Lords held that the judge had wrongly directed the jury that the mistake must be a reasonable one; the correct legal test was whether the defendants had honestly believed the wife was consenting, not whether they reasonably believed this. However, on the facts the House of Lords held the conviction was nonetheless safe despite the misdirection. R v Williams (Gladstone) confirmed the principle stated in Morgan that a belief that a certain set of facts are true does not need to be reasonable to operate under the defence of mistake. It merely needs to be genuine. However, the reasonableness of that belief is material in the jury deciding whether the defendant had actually held that belief.
An exception to this appears to be bigamy (see R v Tolson (1889) 23 QBD 168).
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.
It is not a defence that the defendant held an honest and reasonable belief that what he was doing was not criminal. Where the defendant is a foreigner, and the offence is not criminal in his own country, the fact of such a belief is still not a defence. It is not a defence that the defendant believed that he would not be prosecuted for what he was doing.
In any proceedings against any person for an offence consisting of a contravention of any such statutory instrument, it shall be a defence to prove that the instrument had not been issued by [or under the authority of] His Majesty’s Stationery Office at the date of the alleged contravention unless it is proved that at that date reasonable steps had been taken for the purpose of bringing the purport of the instrument to the notice of the public, or of persons likely to be affected by it, or of the person charged.
(Words in brackets inserted by section 1(1)(a) of the Statutory Instruments (Production and Sale) Act 1996, as read with section 1(2))