Ignorance of the law is no excuse - that is, unless you're a police officer. For the first time, in December, the Supreme Court upheld a traffic stop even where there was no traffic violation. The court, in Heien v. North Carolina, continued its steady erosion of the Fourth Amendment's protection against unreasonable searches and seizures.
In this case, an officer stopped a car that had only one working brake light, thinking that North Carolina law required two working brake lights. But the officer was mistaken about the law. Only one working brake light is required in North Carolina.
Although the court has upheld searches when an officer has made a mistake about the facts, the court has never before said an officer can stop someone due to a mistaken belief the person is committing a crime.