In the 1973 case of Cass v. United States, the Supreme Court ruled on whether a defendant's right to counsel was violated when his attorney failed to object to an incorrect jury instruction regarding intent in a bank robbery trial. The court held that there was no violation because even if the lawyer had objected and received a correct instruction, it would not have changed the outcome of the trial given overwhelming evidence against Cass. This decision established that for ineffective assistance of counsel claims, defendants must show not only that their attorneys' performance fell below an objective standard of reasonableness but also demonstrate prejudice resulting from this deficiency - meaning they need to prove there is reasonable probability that but for counsels’ unprofessional errors, result of proceeding would have been different.

In the dissenting opinion for Cass v. United States, the justice argued that the majority's decision to uphold a conviction based on evidence obtained through warrantless wiretapping was in violation of Fourth Amendment protections against unreasonable searches and seizures. The justice contended that such surveillance techniques constituted an invasion of privacy without proper justification or oversight, thereby undermining fundamental constitutional rights. Furthermore, they disagreed with the majority's interpretation of Title III of the Omnibus Crime Control and Safe Streets Act as permitting this type of surveillance activity; instead arguing it should be read more narrowly to protect individual liberties. They also expressed concern about potential abuse by law enforcement agencies if given unchecked power to conduct electronic eavesdropping without warrants or other safeguards in place.