Police officers, like everyone else, make mistakes. It’s not necessarily rare for one to detain a motorist for a purported violation of law that actually isn’t a violation. For example, an officer might stop someone upon the mistaken belief that a city code requires drivers to have interior rear-view mirrors in their vehicles. (United States v. Chanthasouxat, 342 F.3d 1271 (11th Cir. 2003).)
For those who receive a citation that’s based on an officer’s misunderstanding of the law, the remedy is straightforward: If the driver can establish that the ticket is for something that isn’t a violation, the court will toss it. But what’s a defendant to do when an officer detained her due to a mistake of law, then uncovered evidence of an unrelated crime?
Mistakes of Fact
Most courts distinguish between mistakes of law and mistakes of fact. A mistake of fact occurs when an officer simply gets the facts wrong—for example, by perceiving a crack in a windshield to be bigger than it is. In general, if the mistake is reasonable—for instance, because the windshield crack appeared to be, but wasn't big enough to be illegal—then it’s legally justified. (United States v. Cashman, 216 F.3d 582 (7th Cir. 2000).) In that instance, evidence the officer uncovers after the stop will be admissible. (See Invalid Police Searches and the Good Faith Exception.)
Mistakes of Law
But most courts won’t sanction an officer’s mistaken understanding of the law, no matter how reasonable. To these courts, any discovery following a mistake of law requires suppression of evidence. It’s worth noting, however, that some courts will excuse “reasonable” mistakes of law by police officers. (For information on mistakes of law by defendants, see Can I be convicted of a crime if I didn’t realize what I did was illegal?)
Example: Officers find a gun in the defendant’s car after they stop him for improper use of a turn signal. The defendant had used his signal at a bend in the road, but where there was no place to turn onto a different street. But the relevant law didn’t make it a crime to continue on the same street after activating the turn signal. Since the officers were mistaken as to the law, there was no justification for the stop. The court therefore rules the evidence of the gun inadmissible. (United States v. McDonald, 453 F.3d 958 (7th Cir. 2006).)
Example: Late at night, with no one else around, two police officers observe a car travelling at just over 65 miles per hour in a construction zone that has a posted 60 mile-per-hour speed limit. The general speed limit is 70 mph. The officers pull the car over, at which point they find marijuana on the passenger seat. The officers relied upon a mistaken understanding of the law when they stopped the driver—the law allows motorists to travel over the posted limit in construction zones when no workers are present as long as they stay under the general speed limit. But since the officers’ belief that the driver had been speeding was reasonable, the court refuses to suppress the marijuana evidence. (Harrison v. State, 800 So. 2d 1134 (Miss. 2001).)