On Dec. 15, the U.S. Supreme Court held in Heien v. North Carolina, 13-604, that a traffic stop based on a reasonable mistake of law by a police officer does not violate the Fourth Amendment. This opinion changed the law in most jurisdictions, including in California federal and state courts.Heien raises at least two intriguing questions for lower courts. One was the focus of the concurring opinion: What standard should a trial court apply to determine whether an officer acted reasonably? The second was not directly addressed: Does the good faith exception to the exclusionary rule apply to officer mistakes of law? If that exception applies, it could create for California courts an unusual inquiry involving two closely related steps, wherein a court first determines whether an officer acted reasonably, and then, if not, determines whether she acted in good faith. Heien involved a common scenario where the issue of officer mistake arises. In such cases, an officer stops a suspect for a traffic violation and, in the course of the stop, discovers contraband, leading to a much more serious charge. If the stop later is held invalid because the officer lacked objective reasonable suspicion to conclude that the driver violated a traffic law, then evidence seized during the stop typically must be suppressed under the exclusionary rule. In Heien, a North Carolina officer stopped Nicholas Heien because he had only one working rear brake light. After the officer found cocaine, Heien was convicted by the state for trafficking in the substance. The officer was mistaken about North Carolina law in stopping Heien. The state statute requires that a car be equipped with "a stop lamp," not two, so Heien was not in violation. If, as most courts have held, a mistake of law (whether reasonable or unreasonable) means that a stop is unconstitutional, the cocaine would be suppressed. But, an eight-member majority of the Supreme Court reasoned, the officer's mistake was reasonable. Because of some indications in the statute that all rear lights must be functional, it would have been reasonable (though wrong) for an officer to conclude that the outage of Heien's light was a violation. The Supreme Court held that a stop based upon a reasonable mistake of law was not a Fourth Amendment violation at all. The touchstone of the Fourth Amendment is reasonableness, and because "reasonable men make mistakes of law," such mistakes may be "compatible" with the concept of reasonable suspicion necessary for a constitutional stop. The Heien majority did not explain how a lower court is to determine whether an officer's mistaken belief about the law is reasonable, other than to emphasize that the test is objective and that it "is not as forgiving" as a similar test applied when a mistaken officer asserts qualified immunity in a civil suit. This leaves considerable room to develop a test, as the qualified immunity rule, in the words of a recent Supreme Court case, protects "all but the plainly incompetent or those knowingly violate the law." The question of what reasonableness test to apply may bedevil lower courts in the future. Justice Elena Kagan, however, wrote a concurring opinion joined by Justice Ruth Bader Ginsburg in which she provided a test for reasonableness and emphasized that it is demanding. A legal error can be reasonable only when the law is "so doubtful in construction" that a reasonable judge could agree with the officer's view of it. The statute must be "genuinely ambiguous" and require "hard interpretative work" to resolve the "really difficult" or "very hard question of statutory interpretation" presented. Though the legal mistake by the officer in Heien satisfied the reasonableness test, Kagan predicted that the nature of the inquiry means that rulings for the prosecution on this ground will be "exceedingly rare." It is unclear whether lower courts will apply Kagan's test, which, while adopted by only two justices, is the only test the court provided. Whatever reasonableness test is applied, there is another way that Heien might change the outcome of cases in California courts. This is due to an oddity about Heien. North Carolina is one of only 13 states that - unlike other states and federal law - do not recognize a "good faith" exception to the Fourth Amendment exclusionary rule, as a matter of state law. When that exception applies, it means that evidence from a search is admissible even where there was a Fourth Amendment violation. Because Heien came from North Carolina, whether and how the good faith exception applies to the case was not before the court, which was limited to deciding whether there was a Fourth Amendment violation in the first place. This was to the evident frustration of several justices at oral argument, as much of the argument was spent exploring the import of this unusual posture, where the same case from the federal system would have presented the justices with the option of holding that the cocaine was admissible under the good faith exception to the exclusionary rule, rather than addressing whether there was a Fourth Amendment violation. Unlike the argument, the Heien opinions barely allude to how the good faith exception would impact the case. In her dissent, Justice Sonia Sotomayor took it as a given that the good faith exception generally applies when an officer makes a mistake of law in a stop, arguing that there was little need for the majority's holding there was no Fourth Amendment violation. The Heien majority discussed the exception only insofar as it held that the cases applying that exception did not foreclose consideration of officer reasonableness in the antecedent question of whether there was a Fourth Amendment violation at all. This also leaves lower courts with an issue. Until now, lower courts generally have denied a good faith exception to the exclusionary rule in cases involving officer legal mistakes, but have done so because they have concluded (now erroneously under Heien) there is a Fourth Amendment violation when a stop is based on a reasonable mistake of law. Both conclusions flow from the courts' desire to deter mistakes of law. United States v. Song Ja Cha, 597 F.3d 995, 1005 (9th Cir. 2010); People v. White, 107 Cal. App .4th 636, 644 (2003). This deterrence rationale was unsettled by Heien. Rejecting Heien's deterrence argument, the majority stated "our decision does not discourage officers from learning the law. The Fourth Amendment tolerates only reasonable mistakes, and those mistakes ... must be objectively reasonable." While trial courts likely are bound by the holdings in their jurisdictions that render the good faith exception unavailable, ultimately appellate courts must address whether the Heien reasoning that has undermined their Fourth Amendment violation conclusion also makes the good-faith exception available, as Sotomayor assumed. When a lower court is confronted with a claim that a mistake of law is reasonable, it may first need to determine whether the proper test of reasonableness is the strict one suggested by Kagan's concurrence - whether a reasonable judge could adopt the officer's construction of the law - where a Supreme Court majority did not adopt that test but also offered no different one. If the conduct fails the reasonableness test, whether the good faith exception to the exclusionary rule is available may loom large, particularly if the reasonableness test is stringent. Several reported mistake-of-law cases have concerned California officers stopping out-of-state cars with the erroneous belief they did not meet licensing requirements - such as by having only one license plate, or by not having registration stickers visible from the rear. In such cases, the law of the home state is clear, and no reasonable judge could agree with the officer's interpretation of the state statute. The officers plainly would fail Kagan's test. Yet, it is possible that the officers would be found to have acted in good faith, allowing the fruits of the search as evidence based on a step of reasoning that Heien didn't (and couldn't) address. See United States v. Lopez-Soto, 205 F.3d 1101, 1106 (9th Cir. 2000) (suppressing evidence because officer made a mistake of law due to misplacement of a Baja California registration sticker, though appellate panel had "no doubt" officer acted in good faith.) The unsettled nature of the reasonableness inquiry, and its unsettled interaction with the good faith exception, may mean that, whatever the merits of the solo dissent by Sotomayor in Heien, she may be right that Heien "will prove murky in its application."