In Walder v. United States, the Supreme Court ruled that evidence obtained illegally by law enforcement could be used in court to impeach a defendant's testimony if it directly contradicted their statements under oath. The case involved Albert Walder who was indicted for selling heroin but had his charges dismissed because the drugs were seized unlawfully during an illegal search of his home. Later, he was charged with another drug offense and testified that he never possessed any narcotics. The prosecution then introduced the previously excluded evidence from the unlawful search to contradict his statement on stand. Although this violated what is now known as "the exclusionary rule," which generally prohibits using illegally obtained evidence in court, Justice Felix Frankfurter reasoned that a defendant cannot perjure himself by lying about past criminal behavior and then use constitutional protections against unreasonable searches as a shield.

In the dissenting opinion for Walder v. United States, Justice William O. Douglas argued that evidence obtained illegally should not be used in court under any circumstances, even to impeach a defendant's credibility. He maintained that allowing such use would undermine the Fourth Amendment's protection against unreasonable searches and seizures by providing an incentive for law enforcement officers to conduct illegal searches in hopes of finding incriminating evidence they could later use if needed. Furthermore, he contended that this ruling blurred the distinction between direct and impeachment uses of unlawfully seized evidence which had been previously established by courts as a safeguard against police misconduct. In his view, it was essential to uphold these principles regardless of whether or not they benefited individuals who were guilty because their purpose was primarily to deter unlawful actions by government officials rather than protect criminals.