The U.S. Supreme Court case Cordell Pearson, et al. v. Afton Callahan in 2008 revolved around the issue of whether a police officer's reasonable mistake of law can provide the individualized suspicion that Fourth Amendment requires to justify a traffic stop. The incident began when Officer Morgan stopped Callahan for using his headlights in a way he believed was illegal under Utah law, but it turned out that this interpretation was incorrect and there were no grounds for arrest or search based on headlight use alone. However, during the stop, Officer Morgan found drugs in Callahan’s car and arrested him on drug charges instead. Callahan argued that since the initial reason for stopping him was not valid under state law, any evidence obtained from it should be suppressed as fruit of an unlawful seizure (the "fruit of poisonous tree" doctrine). The court ruled against Callahan stating that while ignorance of law is typically no excuse for citizens who break laws unknowingly; however, if an officer makes an honest mistake about what constitutes legal behavior - such as misinterpreting traffic regulations - then their actions are still considered reasonable enough to stand up in court.

In the dissenting opinion for Cordell Pearson, et al. v. Afton Callahan, Justice Ginsburg argued that the majority's decision to dismiss Callahan's claim was a misinterpretation of clearly established law and an overreach of qualified immunity protections for police officers. She contended that any reasonable officer should have known that they were violating Fourth Amendment rights by entering a home without a warrant or exigent circumstances, as had been previously established in prior case law such as Payton v. New York (1980). Furthermore, she criticized the majority’s approach which required exact factual correspondence with precedent cases to deny qualified immunity; instead arguing it should be enough if the unlawfulness is apparent in light of pre-existing law. Thus, she believed Callahan should have been allowed his day in court to argue his claims against unlawful search and seizure.