In the case of Masciale v. United States, 1957, the defendant John Masciale was convicted for conspiring to violate federal narcotics laws. The conviction was based on evidence provided by an undercover agent who had not been identified or produced at trial. The Supreme Court ruled that this did not infringe upon his Sixth Amendment right to confront witnesses against him as there were other corroborating pieces of evidence supporting his guilt beyond a reasonable doubt. However, it also held that the lower court erred in instructing the jury they could convict solely based on uncorroborated testimony from an accomplice (the informant). Therefore, while affirming Masciale's conspiracy conviction due to sufficient corroborative evidence present in this case, it clarified legal standards regarding reliance on uncorroborated testimony from informants or accomplices.

In the dissenting opinion for Masciale v. United States, Justice Brennan argued that the majority's decision to uphold Masciale's conviction was a departure from established Fourth Amendment jurisprudence. He contended that there was no probable cause for Masciale's arrest and subsequent search, which yielded evidence used against him in court. The only basis for his arrest was an uncorroborated tip from an anonymous informant, which Brennan believed did not meet the standard of "reasonable grounds" required by law enforcement before making an arrest or conducting a search. Furthermore, he criticized the majority’s reliance on post-arrest events to justify their ruling as it could potentially encourage unlawful arrests based on mere suspicion with hopes of later validating them through discovered evidence.