On the morning of April 15, 2013, two California poker players were traveling west on Interstate 80 in Iowa, on the way back from a tournament in Joliet, Illinois, when a state trooper pulled them over. By the time the traffic stop was over, police had seized $100,000 in poker winnings from the two men, on the assumption that the cash must be connected to drug trafficking or some other illegal activity.

In addition to the legalized theft that is civil asset forfeiture, the case illustrates the broad discretion that police have to hassle innocent people, a power magnified by loose rules concerning traffic stops and car searches. Two cases before the Supreme Court could help rein in that power.

Heien v. North Carolina, which the justices heard last week, raises the question of whether stopping a driver for something that's not actually illegal violates the Fourth Amendment's ban on unreasonable seizures. On April 29, 2009, Sgt. Matt Darisse of the Surry County, North Carolina, sheriff's department stopped a car because of "a nonfunctioning brake light." The driver, Maynor Javier Vasquez, and his passenger, Nicholas Heien, the car's owner, consented to a search, which revealed cocaine, resulting in drug charges against Vasquez and Heien. A state appeals court ruled that Darisse had misinterpreted North Carolina law, which does not require two functional brake lights, and that the traffic stop was therefore "objectively unreasonable." The North Carolina Supreme Court disagreed, finding that the stop was reasonable given "the totality of the circumstances."

Police already have so many excuses to pull you over that allowing them to make up new ones seems gratuitous. In the Iowa forfeiture case, for instance, Trooper Justin Simmons claimed that John Newmer­zhycky, who was driving, failed to signal as he passed a black SUV. Footage from the dashcam on Simmons' patrol car contradicts that claim, showing the left signal of the red Nissan Altima driven by Newmer­zhycky flashing as he passes the SUV on the left. In the absence of such contrary evidence, cops are free to invent minor infractions. Or they may decide to act on violations they would otherwise ignore because they do not like a driver's looks or because they hope to find contraband or cash. The Supreme Court has said such pretextual stops do not violate the Fourth Amendment.

There are plenty of small transgressions to choose from, including speeding, following too closely, crossing a solid line, improper passing, improper horn blowing, failing to signal a turn, driving with tinted windows, making a rolling stop, and failing to wear a seat belt (in any of the 33 states with primary enforcement). Why let cops invent more reasons to stop people?

The North Carolina Supreme Court argues that "an officer may make a mistake, including a mistake of law, yet still act reasonably under the circumstances," and "so long as the officer's mistake of law is objectively reasonable…the Fourth Amendment would seem not to be violated." But as George Washington University law professor Orin Kerr points out in a recent discussion of Heien, courts do not cut people without badges that kind of slack. The usual rule is that ignorance of the law is no excuse. Shouldn't police officers empowered to use force against their fellow citizens be held to at least as high a standard?

A Vermont traffic stop that is the focus of a lawsuit filed last month raises similar questions about the constitutionality of detaining people based on a legal mistake. Trooper Lewis Hatch stopped Gregory Zullo, a 21-year-old resident of Rutland, on Route 7 in Wallingford around 3 p.m. on March 6, ostensibly because snow partially obscured the registration sticker on his rear license plate. But as the ACLU of Vermont points out, that is not a traffic violation under Vermont law. In fact, the complaint says, "Mr. Zullo was perfectly obeying all applicable traffic laws when driving through Wallingford that day."

Vermont does require that license plate numbers be visible, which is the rule Hatch cited. But the complaint notes that "Hatch had no difficulty reading Mr. Zullo's rear license plate or the validating sticker affixed to the rear license plate." Was Hatch's erroneous application of the law reasonable? If so, by the North Carolina Supreme Court's logic, so was the stop.

After detaining Zullo for an hour and repeatedly pressing him for permission to search the car, Hatch had the vehicle towed to the state police barracks, saying it smelled of marijuana. Based mainly on that claim and a drug-sniffing dog's purported alert to the car, Hatch obtained a warrant and searched the vehicle, finding a pipe and grinder. The traffic stop did not result in any citations or criminal charges, but Hatch detained Zullo for about an hour before leaving him on the side of the road without transportation. Zullo finally got his car back late that night, seven hours after Hatch stopped him. "To add insult to injury," says ACLU of Vermont Executive Director Allen Gilbert, "the state police made him pay $150 for the tow, as if the situation was his fault."

In a free society governed by the rule of law, people should be reasonably confident that they won't be detained by police unless there is evidence that they have done something wrong. Allowing cops to stop motorists based on misinterpretations of the law tends to undermine that confidence. Making it clear that such stops are unconstitutional would encourage police to be more careful when they presume to hold people against their will.

The Supreme Court recently agreed to hear another case dealing with traffic stops. Rodriguez v. U.S. asks the Court to elaborate on its reasoning in the 2005 case Illinois v. Caballes, where it said police are free to use drug-sniffing dogs during routine traffic stops but suggested stops that are "unreasonably prolonged" to facilitate such inspections would violate the Fourth Amendment. Rodriguez poses the question of how long a delay must be to cross that line.

The case involves Dennys Rodriguez, who was pulled over in 2012 on Highway 275 in Nebraska after he swerved onto the shoulder of the road (to avoid a pothole, according to Rodriguez). Morgan Struble, the Valley, Nebraska, police officer who stopped him, gave him a warning, at which point Rodriguez should have been free to go. But Struble, after unsuccessfully seeking permission to walk a drug-sniffing dog around the car, detained Rodriguez another seven or eight minutes, waiting for a deputy sheriff to arrive. Although Struble had a dog in his patrol car, he did not want to bring it out until he had another officer to back him up. After the deputy arrived, Struble walked the dog around the car, it alerted, and the cops searched the car, finding a bag of methamphetamine.

In January the U.S. Court of Appeals for the 8th Circuit ruled that "the traffic stop was not unreasonably prolonged." The delay of seven or eight minutes, it said, had no constitutional significance, amounting to "a de minimis intrusion on Rodriguez's personal liberty." The fact that the Supreme Court agreed to hear the case suggests at least a few justices may disagree.

If so, it would be a welcome deviation from the deference the justices have almost always shown to police and their canine collaborators. Last year, in Florida v. Harris, the Court unanimously ruled that a police dog's alert, which may be erroneous, imagined, invented, or deliberately triggered, by itself is enough to justify a search unless the defendant can show the dog is unreliable—a tall order when the evidence on that point is controlled by the police, who have little incentive to collect it. (Equating a dog's alert with probable cause is especially problematic given the small percentage of cars carrying contraband, which implies that even a properly trained dog may be wrong more than 80 percent of the time when it indicates the presence of drugs.) Combined with earlier decisions saying police are not conducting a search when they use dogs to detect contraband and therefore can do so without any evidence of criminal activity, Harris in practice gave cops with dogs a license to search vehicles at will. But since not all cops have dogs, there is still a practical limit on this power. That limit will be eroded or eliminated if an officer, after issuing a warning or citation, can continue to detain a driver until a dog arrives.

The Iowa forfeiture case shows the danger of giving police that kind of leeway. Simmons, the trooper who stopped the poker players' car, did not have a dog, so he had to wait for one. As Officer Struble did in Rodriguez's case, Simmons dragged out the encounter after issuing a warning by asking for permission to walk a dog around the car. When Newmerzhycky repeatedly said no, Simmons detained him anyway, ostensibly because Newmerzhycky seemed nervous—an all-purpose excuse to detain anyone stopped by police.

Simmons summoned another trooper, whose dog supposedly alerted to the back of the car, at a point where the animal was conveniently hidden from Simmons' dashcam. In the trunk the troopers found a locked briefcase that belonged to John Davis, the passenger. It contained $85,000. The cops also found Newmerzhycky's computer bag, which contained another $15,000, plus a grinder with bits of marijuana in it, which resulted in a citation for possession of drug paraphernalia—the only Iowa charge brought against either man. If Simmons had let Newmerzhycky go after issuing a warning, there would have been no purported dog alert to justify the search that discovered the money.

Ultimately the state agreed to return $90,000 of the poker players' money, a third of which was consumed by legal fees. In a federal lawsuit filed last month, Newmerzhycky and Davis are seeking the rest of their money, plus compensation for the other costs that resulted from their encounter with Simmons. They argue that the stop, the search, and the seizure were unconstitutional.

This article originally appeared at Forbes.com.