In the case of United States v. Peltier, 1974, the U.S Supreme Court dealt with an issue related to evidence obtained illegally by law enforcement officers. The defendant, Francis Peltier was stopped and searched by police while driving a car that had been reported stolen. During this search, weapons were found in his vehicle which led to charges against him for illegal possession of firearms. He argued that since the initial stop was based on faulty information (the car hadn't actually been stolen), all subsequent evidence should be excluded under the "fruit of the poisonous tree" doctrine - a legal principle stating that any evidence collected or analyzed in violation of a defendant's constitutional rights is considered tainted and must be excluded from trial. The court ruled against Peltier arguing that even if there had been an error made during his arrest process it would not have affected whether or not he possessed those guns at time they were discovered; therefore such exclusionary rule did not apply here because its purpose is to deter unlawful police conduct rather than rectify errors made after-the-fact.

In the dissenting opinion for United States v. Peltier, Justice Brennan argued that the majority's decision to allow evidence obtained in violation of a defendant's Fourth Amendment rights was fundamentally flawed. He contended that this ruling undermined the exclusionary rule which is designed to deter unlawful police conduct by excluding illegally obtained evidence from trial. The majority had justified their decision on grounds that law enforcement officers acted in good faith reliance on existing precedent at the time of search and seizure, but Brennan countered this argument stating it would be impossible to determine whether an officer truly relied upon such precedents when violating a person’s constitutional rights. Furthermore, he emphasized that allowing such exceptions could lead to erosion of Fourth Amendment protections over time as more exceptions are made based on changing legal interpretations or shifting societal norms.