Under the Fourth Amendment to the U.S. Constitution, citizens are protected from unreasonable searches and seizures of their "persons, papers, houses and effects." Reasonableness is considered to exist only when there is "probable cause." Should law enforcement violate this right, the evidence found can be subject to suppression, i.e. not admissible in evidence against you.
This is called the "exclusionary rule." However, what many people do not realize is that this rule is not an individual right; it applies only where, under the circumstances of the case, the court finds that the exclusion of the evidence must be used to deter future violations by law enforcement. This need for deterrence must also outweigh the substantial cost of letting the guilty go free.
This legal point came up in the recent U.S. Supreme Court case of Herring v. United States, in which the defendant was arrested based on a warrant issued by another county. Unbeknownst to the police that were executing the warrant, it had been recalled months before, but the status was never updated in that county's database.
A search incident to that arrest resulted in the discovery of guns and drugs, which were the basis of new charges against the defendant. Herring tried to suppress the evidence because it arose out of an improper and warrantless arrest.
The efforts to suppress were denied by the trial court and affirmed on appeal. When it reached the U.S. Supreme Court, the denial was upheld. In its opinion, the court said,
"To trigger the exclusionary rule, police conduct must be sufficiently deliberate that exclusion can meaningfully deter it, and sufficiently culpable that such deterrence is worth the price paid by the justice system. The pertinent analysis is objective, not an inquiry into the arresting officers' subjective awareness."
Here, law enforcement acted in good faith on what they thought was a valid and current arrest warrant. Under these circumstances, the court did not see deterrence as a substantial enough issue to outweigh the injustice to the public. Law enforcement would have been punished for making an honest mistake.
What the average citizen should take away from this case is that, although the Fourth Amendment protects them, exclusion of evidence based on the lack of probable cause is not an automatic thing. This part of the Bill of Rights protects Americans from unreasonable attempts by the government to violate their privacy, and suppression of evidence only takes place in the face of that unreasonableness. Where law enforcement acts in good faith based on facts that, if true, would give rise to reasonable searches and seizures, evidence is not excluded.