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In general, any error or misconception. 

In contract law, a situation where the parties did not mean the same thing when they agreed to a term or provision.  Also, when at least one contracting party held a belief that was factually or legally false.  As a result, the contract may be subject to modification or rescission.

In criminal law, a defense that claims either a misconception of the law or some fact.  The defense of mistaking the law almost never succeeds.  Mistakes of fact can work as a defense more often, especially if they are reasonable.