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.
[citation needed] 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.
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.
In Australia, the High Court's 2005 ruling in R v Lavender[2] prevents the use of any 'reasonable mistake of fact' defense in cases of involuntary manslaughter.
[3] However, the defense of mistake is available to offences of strict liability such as drunk driving: see DPP v Bone [2005] NSWSC 1239.
[8] Separate legislation has also been used when powers have been transferred from the States to the Commonwealth, such as with the Corporations Act 2001 (Cth)[11] that includes penalties for misconduct by company directors, and in implementing international treaties, such as with the International Criminal Court Act 2002 (Cth)[12] that implemented the Rome Statute into Australian law.
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.