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The Supreme Court has been whittling away at the Fourth Amendment for decades. The Court's 2014 ruling in Heien v. North Carolina allowing the police to make a traffic stop based on a reasonable mistake of law generated little controversy among the Justices and escaped largely unnoticed by the press-perhaps because yet another Supreme Court decision reading the Fourth Amendment narrowly is not especially noteworthy or because the opinion's cursory and overly simplistic analysis equating law enforcement's reasonable mistakes of fact and law minimized the significance of the Court's decision. But the temptation to dismiss Heien as just another small chink in the Fourth Amendment's armor ought to be resisted. The Court's ruling substantially expands police officers' already broad discretion to make traffic stops, including pretextual ones, to now include circumstances where no violation of law even occurred.

Drawing on both criminal procedure jurisprudence and the criminal law literature discussing mistake of law, this Article begins with a critique of the Court's reasoning in Heien. The Article then addresses the potential reach of the Court's ruling. Examining the lower courts' application of Heien in the eighteen months after it was decided, the Article points out that the decision can be read broadly to forgive a wide variety of "reasonable" police mistakes of law. Even more problematic, the cumulative impact of Heien and some of the Supreme Court's other recent Fourth Amendment opinions could potentially lead courts to tolerate even unreasonable mistakes law enforcement officials make in interpreting the law. The Article concludes that, if the Fourth Amendment is construed to allow any mistakes of law, it should borrow from criminal law and ignore police officers' legal errors only when they relied on an official interpretation of the law made by an independent, authoritative third party. Affording the police broader leeway to act based on their own misunderstandings of law is unjustifiable and threatens to further fuel the growing tensions between law enforcement and communities of color.