An interesting Fourth Amendment case
The Supreme Court today handed down Heien v. North Carolina. An officer stopped a car that had a brake light burned out, and in the course of the stop discovered cocaine. It turns out that the North Carolina traffic statutes say that a vehicle must have a working stop "lamp," singular. The State court of appeals ruled that this meant the stop was not supported by probable cause: no reason to believe a law was being violated. The State supreme court ruled that the stop was valid, because the officer's misunderstanding of the law was reasonable. A reasonable mistake of fact does not eliminate probable cause; does a reasonable mistake of law do so?
The Supreme Court ruled 8-1 that the stop was valid. Justice Kagan and Ginsburg concurred, writing to stress that (1) the question of reasonableness is an objective one, not one linked to the officer's personal thoughts or training, and (2) the test is stricter than the one used for qualified immunity (which is loose indeed). Justice Sotomayor dissented, arguing that a mistake of law, reasonable or not, means there is no probable cause.
UPDATE: I quite agree that it is incongruous to have government actors protected by a "reasonable mistake" defense, and have citizens often covered by "ignorance of the law is no excuse." It's particularly so in the area of qualified immunity where, as Sotomayor's dissent points out, civil suit is not allowed unless "anyone but a complete incompetent would have known this was illegal and unconstitutional." And that's when a government actor's pocketbook, not his liberty, is at stake.
With a private citizen, ignorance of the law is no excuse, and even when intent is required the prosecution can ask for a "willful ignorance" instruction, that essentially even if defendant didn't know, if you think he sorta shut his eyes to it, you can find him guilty.
I haven't researched it, but I wonder how this plays out in the setting of a criminal prosecution of a government actor for deprivation of civil rights. Is ignorance of the Constitution no defense, because it's a criminal case, or is reasonable failure to appreciate there was a constitutional right being violated a defense because of qualified immunity? Bear in mind that all of this is judicially created. The statutes say nothing about qualified immunity, or about willful ignorance.