In the 1936 case United States v. Giles, the Supreme Court ruled on a matter involving federal income tax evasion. The defendant, Mr. Giles, was charged with willfully attempting to evade or defeat his income tax for the year 1929 by filing a false and fraudulent return. He appealed his conviction arguing that there was insufficient evidence to prove he had acted "willfully" in evading taxes as required under applicable law at that time. The Supreme Court upheld Mr. Giles' conviction stating that if it could be proven beyond reasonable doubt that an individual knowingly understated their taxable income significantly enough to result in additional tax liability, then this would constitute sufficient proof of "willful" action under existing laws against tax evasion. This decision set important precedent regarding what constitutes 'wilfulness' in cases of alleged federal income tax evasion and clarified how such cases should be prosecuted moving forward.

In the dissenting opinion for United States v. Giles, it was argued that the majority's decision to uphold a conviction based on evidence obtained through wiretapping violated the Fourth Amendment rights of citizens against unreasonable searches and seizures. The dissent emphasized that wiretapping is an intrusive form of surveillance which can potentially expose all private conversations within a person's home or office, regardless if they are related to any criminal activity or not. It was further contended that allowing such practices would set a dangerous precedent where law enforcement could invade personal privacy without any checks or balances in place. Therefore, according to this view, evidence obtained through these means should be deemed unconstitutional and excluded from court proceedings.