Experience can't generally be considered as evidence. For instance, "prior bad acts" of a defendant are usually barred as evidence, unless special circumstances exist. Experience can be taken into consideration in the way that observations are interpreted. This was a major premise in the case of Terry v. Ohio. In this case, a detective observed three men loitering on a street corner on a warm day, all wearing long coats, and occasionally looking through the window of an open business. There was no violation of the law in any of this, but the detective felt, based on his professional experience, that there was a reasonable suspicion that the men were planning to rob the business, and that the long coats were concealing firearms. He stopped them and patted them down for weapons, and recovered firearms from them. They were convicted variously of carrying concealed weapons and being felons in possession of firearms.

Based on the law that existed at this time (1963, although the decision wasn't made until 1968), the detective had no probable cause to stop, detain, and search the men, and absent the decision of the Supreme Court, the conviction would likely have been overturned on the basis of the "fruit of the poisonous tree" doctrine, which prohibits the fruits of an unlawful search, no matter how compelling, to be used in a criminal proceeding. However, the Supremes established new law here and created the "Terry stop" when an officer has an articulable and reasonable suspicion that someone is committing, about to commit, or has committed a crime. The officer's experience is a critical component of the reasonable suspicion finding, as the Supremes recognized that experienced cops are able to detect indications of criminal behavior that may be invisible to private citizens without the same experience.