[dpatru]

Mens rea has to do with the natural consequences of the act while legal mistake (not knowing the law) has to do with the legal consequences. So, eg, if I burn down your house, mens rea asks, "did I intend to cause the fire?" while legal mistake asks, "did I know it was illegal?" Generally lack of mens rea is a defense (I burned down your house by accidently starting a grease fire in the kitchen,) while legal mistake is not (I didn't know it was illegal to burn down houses.) The exception to legal mistake is when the law requires legal knowledge, eg, knowingly failing to file a tax return (I must know that the return is required.)

There is also factual mistake where the mistake is not about the law but about some fact necessary to the crime, eg I intend to link to burn down my neighbor's house but instead, mistaking my house for my neighbor's, burn down my own house.