“There’s not a Barney Fife defense to the violation of the Fourth Amendment,” the legendary advocate Pamela Karlan once told the Supreme Court. The Court disagreed, and held that a police officer had validly arrested a man even though the warrant he relied on had been revoked months before.

Heien v. North Carolina, a case to be argued Monday in front of the Supreme Court, will tell us whether Barney’s loophole is even bigger. Coincidentally, speaking of Barney, this case happened in the hometown of actor Andy Griffith: Mt. Airy, North Carolina, population 10,417.*

On April 29, 2009, Surry County Sheriff’s Deputy Matt Darisse parked by Highway 77 working “criminal interdiction,” a term which seems to mean looking for folks who don’t look right. During his shift, Maynor Javier Vasquez drove by, with the owner of the car, Nicholas Heien, asleep in the back seat.

Darisse became suspicious of Vasquez. It’s a little unclear, why, though: In court, Darisse reasoned that the driver “was gripping the steering wheel at a “10-and-two” position, looking straight ahead”—driving like a regular person, in other words. Darisse followed the car until it came to a stoplight. At that point, he noticed one brake light was out. He stopped the vehicle.

Under the Fourth Amendment, police who want to stop a car need “reasonable suspicion” that someone in it has committed a crime. Once they’ve made a valid stop, they can pull the driver and passengers out for a frisk; bring in drug-sniffing dogs; or ask “consent” to search the car without explaining that the driver has the right to refuse. If permission is refused, they can detain the driver and passengers for hours while they seek a search warrant; and if the driver has committed any offense, even failing to wear a seat belt, they can make an arrest.

That’s the scenario in Heien. Darisse asked Heien for permission to search the car; Heien agreed, and the officers found a baggie full of cocaine.

After Vazquez and Heien were arrested, however, their lawyers made a startling discovery: North Carolina apparently hasn’t fully revised its automobile code since before the days of break lights. Under state law, a car’s only required to have “a stop lamp on the rear of the vehicle.” Yes, “a stop lamp”—not “two brake lights,” as Deputy Darisse and most of the rest of us would assume.

As interpreted by the Supreme Court, the Fourth Amendment creates an exclusionary rule, under which an unconstitutional stop is a “poisonous tree,” and anything that is discovered in a search afterwards is tainted “fruit.” It can’t be used in evidence, and, as then-Judge Benjamin Cardozo wrote, “[t]he criminal is to go free because the constable has blundered.” There are exceptions; there won’t be any exclusion when police make certain kinds of factual mistakes—a warrant that was improperly granted by a judge, for example, or clerical errors in the warrant itself—if the mistakes are reasonable and made in good faith.

Heien asks about the next step: What if the police officer has a “reasonable suspicion” that the driver has done something that turns out not to be against the law? The North Carolina Supreme Court refused to suppress the cocaine, reasoning that the Fourth Amendment exclusionary rule wouldn’t apply. “An officer may make a mistake, including a mistake of law, yet still act reasonably under the circumstances,” the justices held.

But there’s a slight contradiction here. Ignorance of the law is no defense—even if someone makes a “reasonable” mistake. As recently as 1971, the Supreme Court repeated that “[t]he principle that ignorance of the law is no defense applies whether the law be a statute or a duly promulgated and published regulation.” Dozens of lower-court cases since then have reiterated this warning.

Federal and state statutes run to millions of words—they are so complicated that civil-liberties lawyer Harvey Silverglate once wrote that the average American commits three federal felonies a day. If regular people, who lack legal training, are expected to know and follow complicated these laws, is it too much to expect that sheriff’s deputies know the motor-vehicle code they are paid to enforce? Lower courts are split on the issue, but most have held that if an officer makes a mistake about the law, that mistake cannot be used as justification for a stop.

Just last term, the Court held that an anonymous tip that a driver was driving erratically could be grounds for police to stop a car, even if, once police catch up to the car, the driver shows no sign of impairment—a new rule that Justice Antonin Scalia called “a freedom-destroying cocktail.” That case was a line-crossing 5-4: Scalia voted with Justices Ruth Bader Ginsburg, Sonia Sotomayor, and Elena Kagan, while Justice Stephen Breyer joined the conservatives—Chief Justice John Roberts and Justices Anthony Kennedy, Clarence Thomas, and Samuel Alito.

Following this summer’s police shooting in Ferguson, Missouri, there’s a heightened awareness in America that many cops’ gut feelings about whom they should stop are a lot less objective than they often claim after the fact. This case, which shows that driving with your hands at “10 and two” is somehow enough to get a cop on a driver’s tail, will test just how much protection the Fourth Amendment offers to citizens who are guilty or innocent—or just trying to get somewhere.


* This piece originally misstated the population of Mt. Airy, North Carolina. We regret the error.