In the case of Fahy v. Connecticut, 1963, the U.S. Supreme Court ruled that a conviction must be reversed if illegally obtained evidence contributed to the verdict. The defendant, Thomas Fahy and his accomplice were convicted for burglary based on evidence found in their apartment during an unlawful search by police officers who lacked a proper warrant. Although other legally obtained evidence was also used against them at trial, it was determined that the unlawfully seized items had likely influenced the jury's decision to convict. The court held that even if there is sufficient legal evidence to support a conviction, any use of illegal evidence violates constitutional rights under Fourth Amendment protections against unreasonable searches and seizures as well as Fourteenth Amendment due process guarantees. This ruling expanded upon previous decisions which required exclusion of illegal evidence from trials but did not necessarily mandate reversal of convictions where such material had been presented.

In the dissenting opinion for Fahy v. Connecticut, Justice Harlan argued that the majority's decision to overturn a conviction based on improperly admitted evidence was an overreach of federal authority into state court proceedings. He contended that this ruling would lead to unnecessary retrials and could potentially undermine public confidence in the judicial system by suggesting that courts are more concerned with technicalities than with justice. Furthermore, he disagreed with the majority's interpretation of "harmless error" doctrine, asserting it should apply only when there is no reasonable possibility that wrongly admitted evidence contributed to a conviction. In his view, if such evidence might have influenced jurors' decisions even slightly, then its admission cannot be considered harmless and a new trial should be ordered.