In the 1970 case Mackey v. United States, the U.S. Supreme Court ruled on whether evidence obtained through a wiretap authorized by Title III of the Omnibus Crime Control and Safe Streets Act could be used in court if it was collected before that law went into effect but after Katz v. United States (1967) had established an expectation of privacy for telephone conversations. The defendant, Robert Mackey, argued that using such evidence violated his Fourth Amendment rights against unreasonable searches and seizures because at the time of interception there were no statutory guidelines regulating wiretaps as required by Katz ruling. The Supreme Court held in a 5-4 decision that while Katz did require some form of regulation over wiretapping to protect individuals' privacy rights, it did not specify what those regulations needed to be or when they needed to be implemented by Congress. Therefore, since Title III provided these necessary regulations even though it came after the interception occurred, its use was constitutional under retroactive application principles allowing laws clarifying previous ambiguous situations to apply retrospectively.

In the dissenting opinion for Mackey v. United States, Justice Harlan argued that the majority's decision to allow evidence obtained through an unlawful wiretap in a tax evasion case was inconsistent with previous rulings of the Court. He contended that this ruling undermined Fourth Amendment protections against unreasonable searches and seizures by allowing illegally obtained evidence to be used in criminal prosecutions. Furthermore, he expressed concern over how such a precedent could potentially encourage law enforcement agencies to engage in illegal surveillance activities without fear of consequences or penalties. In his view, it was crucial for courts not only to uphold constitutional rights but also deter their violation by excluding unlawfully seized evidence from trials.