Sometimes judges make mistakes when issuing warrants—they may not have had sufficient probable cause to do so. Or, the information relied upon by the police may turn out to be wrong, through no fault of the police. In most situations, the search will still be valid. In U.S. v. Leon (1984), the U.S. Supreme Court ruled that if the police conduct a search in good-faith reliance on a warrant, the search is valid and the evidence is admissible, even if the warrant was in fact invalid through no fault of the police.
Legal Protections Can Vary
Though many critical rights come from the U.S. Constitution, states have their own constitutions and statutes. State law often provides protections that are similar, if not identical, to the those the federal Constitution gives. But occasionally state law offers expanded rights. Keep this potential expansion in mind when reading about general criminal law principles. It could be, for example, that evidence that would be admissible under the federal Constitution is inadmissible under state law.
For more on fundamental rights as they relate to the federal Constitution and state law, see What does the Bill of Rights do?
The Court's reasoning in U.S. v. Leon was that it makes no sense to condemn the results of a search when police officers have done everything reasonable to comply with Fourth Amendment requirements. In addition, the purpose of the rule excluding the results of an invalid search as evidence is to curb the police, not a judge. If a judge makes a mistake, excluding the evidence wouldn't serve any deterrent purpose.
For example, assume that a judge decides that an affidavit submitted by a police officer establishes probable cause to issue a warrant. Even if a reviewing court later disagrees and decides that the warrant shouldn't have been issued in the first place, the officer's search in good-faith reliance on the warrant will be considered valid, and whatever the search turns up will be admissible in evidence. If, however, the warrant is issued on the basis of statements in the affidavit that the police knew to be untrue or made recklessly without proper regard for their truth, the evidence from a search based on the warrant may later be excluded. In this situation, the evidence would be excluded based on the police officer's actions, not an error by the judge.
Example: Officer Furlong searches a residence for evidence of illegal bookmaking pursuant to a search warrant. The officer obtained the warrant by submitting an affidavit containing statements the officer knew to be false. The search is not valid because the police did not act in good faith. Officer Furlong used a false affidavit to obtain the warrant. Whatever the search turns up would not be admissible in evidence.
Example: Officer Cal Ebrate stops a motorist for a traffic violation. A computer check of the driver's license reveals an arrest warrant for the driver. Officer Ebrate places the driver under arrest, searches the car, and finds illegal drugs. It later turns out that the computer record was wrong, and that no arrest warrant actually existed. Here, the officer acted in good-faith reliance on the computer record. Thus, the arrest and search were proper and the drugs are admissible in evidence against the driver, even though the record was wrong (Arizona v. Evans, U.S. Sup. Ct. 1995; Herring v. U.S., U.S. Sup. Ct. 2009).