Mistake of law

Mistake of law is a legal principle referring to one or more errors that were made by a person in understanding how the applicable law applied to their past activity that is under analysis by a court. In jurisdictions that use the term, it is differentiated from mistake of fact.

There is a principle of law that "ignorance of the law is no excuse". In criminal cases, a mistake of law is not a recognized defense, though such a mistake may in very rare instances fall under the legal category of "exculpation". In criminal cases a mistake of fact is normally called simply, "mistake".

Usually, there is in legal cases an irrebuttable presumption that people who are about to engage in an activity will comply with applicable law. As part of the rule of law, the law is assumed to be made available to everyone.The presumption of knowledge of applicable law generally will also apply in the situation of a recent change in the law with which a party in a legal case had no opportunity to become aware of it, e.g. the accused was out hunting in the wilderness and did not know that the law had changed to protect an endangered species.

Some states make a distinction between a mistake as to the substance and effect of existing laws, and a mistake that the law creates a specific right to act in the particular way.Suppose, for example, that A, the owner of a vehicle, takes it into a garage for repair. When returning to collect it, A finds that the vehicle has been left parked in the street. If he has an honest belief that he has the right as an owner to retake possession of the vehicle without paying the outstanding bill for the repairs, he will not be considered as stealing it despite the fact that the garage holds a lien over the vehicle and so has the better right to possession until the bill is paid. This form of the defense is difficult to prove because the defendant must be able to prove that he believed in something more positive than the law permitted the particular behavior. The belief must be that the law creates and vests a specific right to act in that way. Under the Theft Act 1968 and the Criminal Damage Act 1971, a defense will arise if the defendant honestly believes that he is entitled to act in the way he did and this will negate the relevant mens rea element (e.g. of dishonesty under §2 Theft Act 1968). In Chamberlain v Lindon 1998 Lindon demolished a wall to protect a right-of-way, despite allowing nine months to pass before acting, Lindon honestly believed that it was immediately necessary to protect his legal rights without having to resort to civil litigation. For the purposes of §5(2):

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