Mistake of Law. Mistake of Other Law

Traditionally, ignorance of the law is no excuse. If a person steals something, they cannot argue they did not know that stealing was a crime. Relatedly, they cannot argue "mistake of law"--that they were aware of a law--and therefore not ignorant--but that they interpreted the law so as not to apply to them. A person who steals a pen cannot argue that they knew stealing big things was a crime, but didn't realize it applied to small things. 

But there are nuances and exceptions. First, many jurisdictions do recognize some form of mistake of law when the person has relied in good faith upon some official statement of the law that turns out later to be itself mistaken or overruled. We will see an example of this limited defense below in Ostrosky. The defense often arises with those protecting or testing the law for political, religious, or moral reasons. 

Second, mistake of law defenses will depend on which law. The traditional principle applies directly to the law under which the defendant is charged. She cannot claim ignorance of the law she's charged with breaking, such as theft in the above example. 

But sometimes a person will be mistaken about some other law, often a non-criminal law, that itself must be interpreted to determine whether the criminal law charged applies. Put another way, sometimes an element of the charged crime itself invokes some other law. In this way, the element is an attendant circumstance just like any other. That means, at least under the MPC, the prosecution must prove mens rea with respect to that element--even though it involves awareness or interpretation of some other law. 

Both Rehaif and Varsegi involve this version of mistake of law--which is really mistake of other law.