In United States v. McDonald, 453 F.3d 958, 961 (7th Cir. 2006), the Seventh Circuit Court of Appeals noted that "the majority of circuits to have considered the issue" had concluded that "a police officer's mistake of law cannot support probable cause to conduct a stop."
The traffic stop in McDonald was based upon the defendant's use of a turn signal.
The court concluded that the arresting officer was mistaken in his belief that using the turn signal at "a ninety-degree curve in the road" was a violation of the law.
The court held that the traffic stop was improper, and explained, id. at 961-62:
An officer cannot have a reasonable belief that a violation of the law occurred when the acts to which an officer points as supporting probable cause are not prohibited by law.
It makes no difference that an officer holds an understandable or "good faith" belief that a law has been broken. Whether the officer's conduct was reasonable under the circumstances is not the proper inquiry. . . . A stop based on a subjective belief that a law has been broken, when no violation actually occurred, is not objectively reasonable.
Even though the officer may have acted in good faith, there is no good faith exception to the exclusionary rule when, as here, an officer makes a stop based on a mistake of law and the defendant is not violating the law.