My primary emotion when I think of law-enforcement officers is profound gratitude that they did not shoot me to death in my basement rumpus room ten years ago. If they had, it would have been terrible for me, but I have no doubt it would have been a “good shoot.”

What happened was this: After 24 hours of flight delays, I staggered back into my house in Eugene, Oregon, at about 2:30 am. I am not sure I have ever been as tired in my life as I was that night. I dropped my bags and rushed downstairs to the basement guest room, which was dark and quiet and where I could be sure I would not be woken by the sun or morning traffic on the street outside. I flopped on the bed and fell asleep at once.

Unfortunately, in my haste to crash, I had left my front door open.

Three hours later, the newspaper carrier came to my door and found it hanging open. In an act of genuine good citizenship, the carrier called the police to alert them that there might be a crime in progress at my address. (Eugene at that time had a serious and violent burglary problem, fueled by rampant meth use.) The police arrived. They hammered on the door, announced “Eugene Police!” several times, and finally entered. At this point they had reason to suspect that a home invasion or robbery might be in progress.

I slept through the whole thing. But a few minutes later, I stirred and came out of the bedroom to use the bathroom. At once I heard, “Freeze! Put your hands where I can see them!” (That, at any rate, was the gist of what I heard.) I will never forget seeing two officers, guns drawn, confronting me in the den.

It was a scary moment for me. But it must also have been terrifying for them: After loudly announcing themselves, they had moved into what might be a crime in progress to find a man walking around in the half-darkness.

Fortunately, I was too tired to be jumpy or loud. I slowly raised my hands and echoed Eddie Murphy’s immortal words in Trading Places: “Is there a problem, officers?”

Had I reacted differently; had I mistaken the police for burglars themselves; had I had a firearm in the house; or had I been a different person—one perceived by police in the dim light as in some way threatening—I would be dead. And I doubt that anyone would have said the police acted unreasonably.

I owe my life to a mixture of police professionalism and blind luck.

The lessons I draw from this are complex. I think that most police genuinely do not want to use deadly force. But I also think that, in a heavily armed and violent society, any encounter between police and the public is fraught with tension and ambiguity, and can easily go wrong. I don’t know whether Chief Justice John Roberts has ever had a pistol pointed at him by a police officer. I tend to doubt it. I sort of wish that he and the other members of the Supreme Court had found themselves, at least once, in a police encounter that could have gone sideways so easily.

If they had, I suspect, they might not be as cavalier about the problems of motorists stopped by police because of a “reasonable mistake of law.” The holding of Monday’s opinion in Heien v. North Carolina was that a police officer may stop a citizen and question him or her on “reasonable suspicion” of violating the law—even when the “law” the officer is enforcing turns out not to exist.

The facts of Heien are that a North Carolina sheriff’s deputy decided that a passing car was suspicious. The driver, he decided, seemed “very stiff and nervous” because he was looking straight ahead and holding his hands at the recommended positions on the wheel. (I am sure there was no connection, but the driver was also a Latino in an overwhelmingly white county.) The deputy followed the car, seeking a reason to make a stop, until the driver put on the brakes for a red light. One of the two brake lights was out. The deputy pulled over the car for the broken brake light and questioned both the driver and the owner, who had been sleeping in the back seat. Eventually he got permission to search the car, found cocaine, and arrested both men. A fairly open-and-shut case—except that, a state appeals court decided, North Carolina law only requires one working brake light. The “offense” leading to the stop was no more illegal than hanging a pine tree air freshener from the rear-view mirror.

The lower courts refused to suppress the evidence. It is settled law that when an officer makes a reasonable mistake of fact—concludes from appearances that, say, an assault is going on when two friends are just tussling—a stop doesn’t violate the Fourth Amendment. But, Heien argued, a mistake of law is different. Consider the reverse scenario: If North Carolina law did require two brake lights, Heien could not have avoided a ticket by pleading that he thought it only required one. Most of the time, as we all know, ignorance of the law doesn’t get a citizen off the hook.

The Supreme Court had never decided this issue. On Monday, by 8-1, it concluded that the stop was “reasonable.” One can certainly sympathize with the deputy in this case: The North Carolina motor vehicle code on this point is virtually opaque, and the one-brake-light rule wasn’t clear to anybody until the appeals court decided it in Heien’s case. As for the “ignorance of the law” argument, the Chief Justice breezily responded, that’s fine. The deputy didn’t give Heien a ticket for having one brake light. “Heien is not appealing a brake-light ticket,” the Chief wrote. “[H]e is appealing a cocaine-trafficking conviction as to which there is no asserted mistake of fact or law.”

Justice Elena Kagan, joined by Justice Ruth Bader Ginsburg, wrote separately to attempt to limit the effect of the decision. It’s not a question of whether he actually knew the law, but of whether the law was really clear to everybody, she wrote. “If the statute is genuinely ambiguous, such that overturning the officer’s judgment requires hard interpretive work, then the officer has made a reasonable mistake,” she wrote. “But if not, not.” All very well, but I can’t help concluding that Heien makes it easier for police to find a reason to stop anyone they think looks suspicious. And we as a society are learning some very hard lessons about what can go wrong with police stops. Roberts’s opinion takes not the slightest notice of the events of the past year. The world he describes is a kind of happy valley were police are polite, citizens know their rights, consent to search is always freely given, and only evildoers feel dread when they see a blue light in the rear-view mirror. “[R]easonable men make mistakes of law,” as well as of fact, he says.

If only we all lived in the Chief’s empire of reason, and drove on his celestial streets! Those of us in the sublunary world, however, traverse streets where fear, not reason, is often the currency. Justice Sonia Sotomayor, in a solo dissent, protested that the decision “means further eroding the Fourth Amendment’s protection of civil liberties in a context where that protection has already been worn down.” She pointed out that “[g]iving officers license to effect seizures so long as they can attach to their reasonable view of the facts some reasonable legal interpretation (or misinterpretation) that suggests a law has been violated significantly expands [their] authority.” And setting out a standard that permits stops based on “reasonable” errors but not on “unreasonable ones,” she argued, further confuses the world of criminal procedure. Even a citizen who knows the law and conforms to it may be subject to police seizure of the officer “reasonably” misunderstands that same law. The Court’s new standard, she wrote, “will prove murky in application.”

More ambiguity in the law, of course, means more ambiguous police stops. Encounters with law enforcement are murky enough already, for good reasons and bad. Barely a week goes by that I don’t think of, and mentally thank, the good cops who spared my life. But how much better it would have been for all of us if I had just remembered to close the front door. Just so, courts are in business to instruct the state about lines it should not cross. The Court this week passed up a chance to do that.