Police officers can make mistakes, which, for better or worse, the U.S. Supreme Court has often seen fit to forgive. Police, for instance, can make mistakes of fact when assessing whether circumstances justify the seizure of an individual or search of a residence; they can even be mistaken about the identity of those they arrest. This essay examines yet another, arguably more significant context where police mistakes are forgiven: when they seize a person based on their misunderstanding of what a law prohibits. . . .