In United States v. Pacheco, the Supreme Court held that a defendant could not be convicted of an offense based on evidence obtained through an illegal search and seizure. The case arose when federal officers searched the home of Francisco Pacheco without a warrant or probable cause to believe he had committed any crime. During their search, they found documents which were used as evidence against him in court for violating customs laws by importing goods from Mexico without paying duties owed to the government. The Supreme Court ruled that this was unconstitutional because it violated his Fourth Amendment right against unreasonable searches and seizures, thus overturning his conviction and setting precedent for future cases involving similar issues.

In United States v. Pacheco, the Supreme Court was asked to decide whether a federal statute that allowed for the seizure of property used in aiding or abetting an insurrection against the government could be applied retroactively. The majority opinion held that it could not and found in favor of Pacheco. In dissent, Justice Field argued that Congress had acted within its constitutional authority by enacting such a law and noted further that there was no reason why it should not be applied retroactively since this would serve as a deterrent to those who might consider engaging in similar activities in the future. He also pointed out that if Congress were prohibited from applying laws retrospectively then any act committed prior to their enactment would go unpunished regardless of how serious they may have been.