Mistake of law
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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.
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.  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):
it is not necessary to decide whether Lindon’s action was justified as a matter of civil law. For the purpose of the criminal law, what matters is whether Lindon believed that his actions were reasonable, i.e. a subjective test.
Thus a lawful excuse may be acknowledged by a court to arise when a person honestly but mistakenly believes that the actions are necessary and reasonable.
Mistake of non-governing law in the United States
One narrow area of exception occurs where a person makes a mistake of non-governing law. While the accused are not pardoned for failure to know what acts have been deemed criminal, they may not be held to know of non-criminal provisions that affect the status of things that might therefore be deemed criminal. For example, suppose Jennifer is married to Phillip, but decides to get a divorce in order to marry Ben. However, Jennifer mistakenly believes that the divorce was final when she submitted the paperwork required by the state, and did not realize that she had to wait for a court to pronounce her divorced. In the interim, she marries Ben, and so is technically committing bigamy because she has married a second man before her divorce from the first was complete. Jennifer's mistake was not one of governing law (she did not mistakenly believe it was legal to be married to two people), but rather a mistake of non-governing law, which is akin to a mistake of fact. Depending on the jurisdiction in which the act took place, Jennifer may be allowed to raise the defense of mistake of law in such a scenario. See Long v State, 44 Del. 262.
- Richard G. Singer, John Q. La Fond, Criminal Law (2010), p. 98-99.
- Richard G. Singer, John Q. La Fond, Criminal Law (2010), p. 104.