When the U.S. Supreme Court created the Fourth Amendment exclusionary rule in Weeks v. U.S. (1914) and began applying it to the states in Mapp v. Ohio (1961), the theory was that by suppressing evidence gained by unreasonable searches and seizures, courts would deter police officers from violating the Fourth Amendment.
What the exclusionary rule has actually meant in practice is that thousands (maybe millions) of criminals have been able to stop the prosecution from using critical evidence of their guilt to hold them accountable for their crimes. In the famous words of a 1926 court decision, "The criminal goes free because the constable has blundered." (People v. Defore)
In the years since the rule was first announced, the Supreme Court has acknowledged that in some circumstances, suppression of evidence would not deter police misconduct. In such cases, the exclusionary rule is not to be invoked. Examples include good-faith mistakes in obtaining search warrants (U.S. v. Leon) and in serving them (Maryland v. Garrison; Hudson v. Michigan), enforcement of statutes later invalidated by court rulings (Michigan v. DeFillippo; Illinois v. Krull), reasonable reliance on someone's apparent authority to consent to a search (Illinois v. Rodriguez), and misidentification of a person named in an arrest warrant (Hill v. California).
Suppression is also inappropriate if the evidence was derived from an "independent source," would have been located by "inevitable discovery," or followed an intervening act that "attenuated the taint" from an earlier constitutional violation.
The Supreme Court has added one more situation to the list of exceptions to the exclusionary rule: where officers reasonably rely on information coming through "official channels" and that information turns out to be erroneous, it is not necessary to suppress resulting evidence, since there is no misconduct to deter. The court has considered mistakes originating from court personnel and from law enforcement personnel.
Arizona v. Evans
Phoenix officers stopped Isaac Evans for a traffic violation. Using their mobile computer terminal, they entered Evans' name and DOB into a database that showed an outstanding arrest warrant. When they searched his car incident to arrest, officers found drugs. Evans was subsequently prosecuted on the drug charge. But he moved to suppress the evidence, because the arrest warrant on which he was arrested had in fact been quashed before his arrest. Through clerical error, court personnel had failed to correct the database entry showing an outstanding warrant.
The trial judge ordered suppression, and the Arizona Supreme Court upheld this order on Fourth Amendment grounds. The State appealed to the U.S. Supreme Court, arguing that excluding the evidence of Evans' guilt could not deter clerical error in the judicial branch of government and that the police were not guilty of any misconduct for relying in good faith on information received through official channels.
The U.S. Supreme Court agreed and reversed the suppression order, holding that the exclusionary rule should not be applied in such circumstances. Said the court: "The exclusionary rule was historically designed as a means of deterring police misconduct, not mistakes by court employees. The threat of exclusion of evidence could not be expected to deter such individuals from failing to inform police officials that a warrant had been quashed. There is no indication that the arresting officer was not acting objectively reasonably when he relied upon the computer record. [Our precedents support] a categorical exception to the exclusionary rule for clerical errors of court employees." (Arizona v. Evans)