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.
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
Historically, courts distinguished 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 a resulting detention is legally justified. (United States v. Cashman, 216 F.3d 582 (7th Cir. 2000).) In that instance, evidence the officer uncovers after the stop will probably be admissible. (For a related concept, see Police Searches and the Good Faith Exception.)
Mistakes of Law
Traditionally, many courts wouldn’t sanction an officer’s mistaken understanding of the law, no matter how reasonable. To these courts, any discovery following a mistake of law required suppression of evidence. But in late 2014, the U.S. Supreme Court decided that reasonable mistakes of law can give rise to the reasonable suspicion that justifies a detention. In the case in question, an officer wrongly believed that a driver's having only one working brake light violated state law. He detained the driver and the driver's passenger on this basis, and ultimately discovered cocaine in the car. The cocaine led to trafficking charges against the passenger. The Supreme Court found that the officer's misunderstanding of the law was reasonable given the relevant statutes, and that the cocaine was therefore admissible evidence. (Heien v. North Carolina, 574 U. S. ____ (2014).)
For much more on the case, including the relevance of state law in "mistake of law" situations, see Police Detentions Based on Mistakes of Fact and Mistakes of Law.