Mistake of Law Exception

Read About the Exceptions to the Mistake of Law


Regarding the Mistake of Law Exception, in U.S. v. Twilley (9th Cir 2000) 222 F3d 1092, the Court held that a cop’s good faith mistake of law about the number of license plates issued by the state of Michigan did not make the vehicle detention legal. The opinion held that a mistake of law, even if made in good faith, can never justify a detention. The opinion stated:

But in this circuit, a belief based on a mistaken understanding of the law cannot constitute the reasonable suspicion required for a constitutional traffic stop. “Reasonable suspicion is formed by specific, articulable facts which, together with objective and reasonable inferences, form the basis for suspecting that the particular person detained is engaged in criminal activity.” Lopez-Soto, 205 F.3d at 1105. In Lopez-Soto, a police officer stopped the defendant’s car because he could not see a registration sticker on the rear of the vehicle. Although the officer believed that such a visible registration sticker was required by law—and had been so instructed at the police academy—the law actually required the sticker to be affixed to the windshield. The officer, like Officer Weeks in this case, stopped the car “because he held a mistaken view of the law.” Id. at 1105. A suspicion based on such a mistaken view of the law cannot be the reasonable suspicion required for the Fourth Amendment, because “the legal justification [for a traffic stop] must be objectively grounded.” Id. (quotations omitted). In other words, if an officer makes a traffic stop based on a mistake of law, the stop violates the Fourth Amendment.

While the officer need not perfectly understand the law when he stops the vehicle, his observation must give him an objective basis to believe that the vehicle violates the law. We recently upheld the stop of a vehicle when the officer correctly believed the car’s window tinting violated the law, although the officer believed all tinting was illegal and the law actually prohibited only darker tinting. See Wallace, 213 F.3d at 1120 (the officer’s “observations correctly caused him to believe that Wallace’s window tinting was illegal; he was just wrong about exactly why”). This case is “in sharp contrast” to one “in which the [driver’s] conduct does not in any way, shape or form constitute a crime.”

Id. Footnote 1 of the opinion distinguishes the law on a cop’s mistake of fact, which can justify a detention if made in good faith.


Similarly, the opinion in U.S. v. Lopez-Soto (9th Cir 2000) 205 F3d 1101, reaches the conclusion that a good faith mistake in the law regarding display of a vehicle registration sticker does not justify a vehicle stop. The opinion makes it clear that no mistake in law, whether made in good faith or not, can justify any detention, saying:

We have no doubt that Officer Hill held his mistaken view of the law in good faith, but there is no good-faith exception to the exclusionary rule for police who do not act in accordance with governing law. See U.S. v. Gantt, 194 F.3d 987, 1006 (9th Cir. 1999). To create an exception here would defeat the purpose of the exclusionary rule, for it would remove the incentive for police to make certain that they properly understand the law that they are entrusted to enforce and obey.

The California cases generally support the proposition that a mistake of law will not validate a stop. People v. Hernandez (2003) 110 Cal App.4th Supp. 1—officer’s mistaken belief that a neon purple light ringing a front license plate frame was illegal, did not make the stop legal due to the officer’s mistake of law. See also People v. White (2003) 107 Cal.App.4th 636—officer erroneously believed that a car from Arizona was required to have both a front and rear license plate as is required in California. The court wrote at pages 643-44: “Though we assume the officer acted in good faith, there is not good faith exception to the exclusionary rule for police who enforce a legal standard that does not exist. Creating a good faith exception here would run counter to the exclusionary rule’s goal by removing an incentive for the police to know the law we entrust them to enforce.” People v. Ramirez (2006) 140 Cal.App.4th 849 (Officer’s detention of an individual crossing a street outside of a sidewalk was a mistake of law as the officer could not establish that the defendant failed to yield the right of way to a vehicle constituting an immediate hazard. The Court held that detention was illegal because there was no probable cause to believe that a violation occurred.).

For a contrary holding probably limited to the rare facts in the case see People v. Glick (1988) 203 Cal.App.3d 796. The conflict between White and Glick was addressed in People v. Reyes (2011) 196 Cal.App.4th 856, which found Twilley and White to be better reasoned and more in line with established mistake of law cases in California. The court noted: “[T]o hold otherwise would provide a strong incentive to police to remain ignorant.” Id. See Bryan v. United States (1998) 524 U.S. 184, 196 (“We note the fundamental unfairness of holding citizens to the traditional rule that ignorance of the law is no excuse while allowing those entrusted to enforce the law to be ignorant of it.”).

Blog Entry: Mistake of Law Exception (Peter F. Iocona, Attorney at Law)