Mistakes of fact have rarely been an adequate defense at
common law. In the United States, 37 states have ruled out mistake of fact as a defense to charges of attempt. Another case involving the defense of factual impossibility is the
Supreme Court of Pennsylvania's decision in
Commonwealth v. Johnson, 167 A. 344, 348 (Pa. 1933), in which a wife intended to put
arsenic in her husband's coffee but by mistake added the customary sugar instead. Later, she felt repentant and confessed her acts to the police. She was arrested, tried, and convicted of
attempted murder.
Mistakes of law An act that is considered legally impossible to commit is traditionally considered a valid defense for a person prosecuted for a criminal attempt. An attempt is considered to be a "legal" impossibility when the defendant has completed all of his intended acts, but those acts fail to fulfill all the required
common law elements of a crime. Mistake of law has proved a successful defense. An example of a legally failed attempt is a person who shoots a tree stump; that person can not be prosecuted for attempted murder as there is no manifest intent to kill by shooting a stump. The underlying rationale is that attempting to do what is not a crime is not attempting to commit a crime. However, "legal" and "factual" mistakes are not mutually exclusive. A borderline case is that of a person who shot a stuffed deer, thinking it was alive. That person was originally convicted for attempting to kill a protected animal out of season, but in a debatable reversal, an appellate judge threw out the conviction on the basis that it is no crime to shoot a stuffed deer out of season. == Facts of the case ==