Under the Common Law, if the crime only requires general intent, the defendant’s mistake must be reasonable for mistake of fact to be a valid defense. If the crime is a specific intent crime, the defendant’s mistake can be reasonable or unreasonable. This reasonableness is first a legal conclusion, then it goes to the jury to determine whether or not the mistake was reasonable.
Under the MPC Section 2.04, the defendant’s mistake must go towards negating the mens rea required for the crime. If the crime only requires general intent, the mens rea is going to default to recklessness.
See Regina v. Prince for an example of mistake of fact.
Compare to Mistake of Law