In the case of Michigan v. DeFillippo, the U.S. Supreme Court ruled in favor of Michigan, upholding a police officer's right to arrest an individual based on a law that was later deemed unconstitutional. The defendant, DeFillippo, had been arrested for violating a Detroit ordinance requiring individuals to provide identification upon request by police officers; however, this ordinance was subsequently found to be unconstitutional. Despite this fact, during his arrest and subsequent search by the arresting officer illegal drugs were discovered leading to additional charges against him. In court he argued that since the original reason for his arrest (the ID requirement) was invalid so too should be any evidence gathered as result of it (the drugs). However,the Supreme Court disagreed stating that at time of his arrest and search the law had not yet been declared unconstitutional thus making both actions valid under existing laws at that time.

In the dissenting opinion for Michigan v. DeFillippo, Justice Brennan argued that the Fourth Amendment requires police to have probable cause not only that a crime has been committed but also that it is in fact a crime under law. He disagreed with the majority's view that an officer's reasonable belief of legality was sufficient justification for arrest and subsequent search. In this case, he believed there was no probable cause because Detroit’s ordinance requiring identification from individuals suspected of criminal behavior had already been declared unconstitutional by state courts before DeFillippo’s arrest occurred. Therefore, according to Brennan, any evidence obtained as a result should be excluded as fruit of an unlawful arrest. The justice expressed concern about potential abuses if officers were allowed to make arrests based on laws they merely believe are valid without having certain knowledge or checking their validity.