Bruce Schneier has an entry on his security blog about how the government has been telling pilots that they can't fly around nuclear power plants, but refuses to say where those are. So one could get forced to land and probably arrested for something that one didn't know and therefore couldn't have prevented!

Very similar to how the government says that "Ignorance of the law is not an excuse", despite the fact that no one, inside or outside of the government, knows all of the law.