State v. McElroy
Supreme Court of Arizona
625 P.2d 904 (1981)
Deputies questioning Marvin McElroy (defendant), one of two people loitering suspiciously at night, found a bag of white pills in his shirt; McElroy repeatedly insisted the pills were amphetamines, even after being read his Miranda rights. A chemist later determined the pills were not amphetamines or any controlled substance at all. McElroy was nonetheless convicted of attempted possession of dangerous drugs, and he appealed, arguing that since the pills were legal, it was factually impossible for him to have committed the underlying offense.
Whether factual impossibility — where completing the intended act would not actually have been criminal because of an unknown fact — is a defense to a criminal attempt charge.