Ordinarily, ignorance of the law is not an excuse. If you believe that it is permissible to turn right on a red light but the city ordinance prohibits it, your belief, even if reasonable, does not excuse your violation of the law. Under certain circumstances, however, ignorance of law will be excused. If a statute imposes criminal penalties for an action taken without a license, and if the government official responsible for issuing the license formally tells you that you do not need one (though in fact you do), a conviction for violating the statute cannot stand. In rare cases, a lawyer’s advice, contrary to the statute, will be held to excuse the client, but usually the client is responsible for his attorney’s mistakes. Otherwise, as it is said, the lawyer would be superior to the law.
Ignorance or mistake of fact more frequently will serve as an excuse. If you take a coat from a restaurant, believing it to be yours, you cannot be convicted of larceny if it is not. Your honest mistake of fact negates the requisite intent. In general, the rule is that a mistaken belief of fact will excuse criminal responsibility if (1) the belief is honestly held, (2) it is reasonable to hold it, and (3) the act would not have been criminal if the facts were as the accused supposed them to have been.