We all make mistakes---even criminals. However, suppose someone who thinks that what he is doing is legal turns out to be mistaken, and the act is a crime. Is he guilty? The common law answered this question as it often does: “It depends.” Consider a factual mistake. As a general rule, if Angelica reasonably thinks the white powder in her vial is salt though it is really cocaine, she is not guilty of transporting cocaine. The law treats legal mistakes, however, strikingly differently. Arthur has been told by a state EPA director that he may, without a permit, dump what he knows to be toluene. The advice turns out to be a misinterpretation of the environmental statutes. Such a mistake would never exculpate. This tension between legal and factual mistakes and the exceptions to these general rules create ulcers in law students---not to mention in clients.