The U.S. Supreme Court heard oral argument today in Heien v. North Carolina, a case which asks whether a police officer's "mistake of law" can provide the necessary justification for a traffic stop under the Fourth Amendment.

The case arose in 2009 when North Carolina police stopped Nicholas Heien for driving with one broken brake light. That stop led to a search of Heien's vehicle, which in turn led to the discovery of illegal drugs. However, according to North Carolina law, motor vehicles are required only to have "a stop lamp." That means the officer was mistaken about the law and had no grounds for the original traffic stop. Did the officer violate Heien's right to be free from unreasonable search and seizure?

"The government should be presumed to know the laws," declared Jeffrey Fisher, the lawyer representing Heien before the Supreme Court. "It would undercut public confidence in law enforcement and the common law rule upon which the criminal law is built to say the government doesn't have to be presumed to know the law when it acted."

Robert Montgomery, the senior deputy attorney general of North Carolina, took a different approach, urging the justices to grant the police wide leeway—even in those instances when the officers may have acted in error. "The Fourth Amendment prohibits unreasonable searches and seizures, but it does not require that police officers be perfect," Montgomery told the Court. "Because the touchstone of the Fourth Amendment is reasonableness, all that is required is that a police officer have a reasonable view of the facts and apply those facts to a reasonable understanding of the law."

Both sides came in for sharp questioning from the bench. Justice Samuel Alito, for example, repeatedly pressed Fisher on why the police officer's approach should not receive more deference. Justice Sonia Sotomayor, by contrast, emerged as a critic of the North Carolina police force. "How many citizens have been stopped for one brake light who are asked to have their car searched?" Sotomayor asked the state lawyer at one point. "And is that something that we as a society should be encouraging?"

Chief Justice John Roberts, meanwhile, struck something of a balance, suggesting that while reasonable mistakes of law by the police can and will occur, the problem with the state's case was its "very broad definition of reasonable." "It sounds to me," Roberts said, "like you're adopting the same standard that we apply in qualified immunity, which gives the officers quite—quite broad scope, and that—that's troubling."

A decision in Heien v. North Carolina is expected by early next year.