The Supreme Court today decided Heien v. North Carolina, No. 13-604, involving the propriety under the Fourth Amendment of a police search.  The Heritage Foundation aptly describes the decision this way:

The Fourth Amendment prohibits (inter alia) "unreasonable" searches and seizures.  For the past 50 years, the Supreme Court has crafted rules for law enforcement officers to follow, in order to make it easier for the police to know what is and is not reasonable.  Additionally, the Court has construed that term to allow the police to make reasonable mistakes of fact, reasoning that the "probable cause" necessary to effect a search or seizure does not require an officer to be absolutely right, just "reasonable."

Today, Roberts, writing for an 8-1 Court, ruled that the term "reasonable" includes reasonable mistakes of fact and law.  An officer, who mistakenly but reasonably believed that the driver of a vehicle had violated a state traffic law by having only one working brake light, stopped the vehicle and ultimately found cocaine, which was used to convict the driver and passenger.  Because the officer's interpretation of the traffic law was reasonable, Roberts concluded, there was no Fourth Amendment violation.  Kagan wrote a separate opinion, joined by Ginsburg, to emphasize the narrowness of the Court's opinion.  Sotomayor dissented on the ground that, regardless of how the exclusionary rule should be applied, there is no "mistake of law" exception to the Fourth Amendment.