Recap: Heien said that a police seizure based on an objectively reasonable mistake of law does not violate the Fourth Amendment. The police had stopped a car because one of the tail lights was not working. As it turns out, state law only required one working lamp. But this was a reasonable, and forgivable, mistake of law, according to eight justices.
How do we work with this? First, don't resign that the issue is dead. This is not carte blanche for police to claim mistakes. Deliberate ignorance will not be tolerated. The Court promised. "The Fourth Amendment tolerates only reasonable mistakes, and those mistakes—whether of fact or of law -- must be objectively reasonable." Hold to the language of Heien. "Thus, an officer can gain no Fourth Amendment advantage through a sloppy study of the laws he is duty-bound to enforce."
Contrast this to Whren, even though the Heien Court does not. The Whren Court allowed a stop for a traffic violation, even when the true subjective motivation for the stop was not enforcing traffic safety but an excuse to come into contact with a drug suspect. Here, Chief Justice Robert's invokes Whren to support the objective standard. "We do not examine the subjective understanding of the particular officer involved." But what if there is evidence that the police officer actually knew the law, however obscure? Can the government still claim "objective" mistake? Justice Kagan probably precludes this argument. Her "objectively reasonable" standard cleaves to honest mistakes, not dissembling claims of objective mistake. "That means the government cannot defend an officer’s mistaken legal interpretation on the ground that the officer was unaware of or untrained in the law."
Next, enforcement of the Fourth Amendment by excluding evidence is not merely to punish or deter police misconduct, and Chief Justice Roberts allows this. The Fourth Amendment should protect against unreasonable police intrusions, and that protection should extend farther and deeper than just a prophylactic effect on police. This isn't just about the police; it is about a person's right to be "secure . . . against unreasonable searches and seizures."
Finally, embrace Justice Sotomayor's defense of the Fourth Amendment. The majority opinion "means further eroding the Fourth Amendment’s protection of civil liberties in a context where that protection has already been worn down." This language can be used in tandem with Justice Kagan's "important limitations" required by Heien.
There is much to guard against. Taken to its legal and logical extreme, Heien will allow both mistake of law and mistake of fact in the same scenario. "Reasonable suspicion arises from the combination of an officer’s understanding of the facts and his understanding of the relevant law." Let's say, hypothetically, it is illegal to drive a green car, but only in the passing lane. Police see a person driving a blue car in the right hand lane and stop the car to investigate their reasonable suspicion of a crime. The police made a mistake of fact (the car was not green, but blue) and a mistake of law (the law only prohibited driving in the passing lane). Under Heien, that's called safe.
Let's throw in the Whren factor -- as long as police are objectively reasonable about the justification for the stop, their subjective motivation is irrelevant. Where are we now? The subjective motivation for stopping the car is irrelevant as long as there is an objectively reasonable basis for the stop, even if that objectively reasonable basis is an objectively reasonable mistake about what law was violated and a mistake about whether the observed facts broke the non-existent law. This truly strains the already thinning protection of the Fourth Amendment. The mistake of fact and law scenario is an illogical extension not likely tolerated by SCOTUS, given the caution recommended by Justice Kagan and the criticism of Justice Sotomayor.