United States vs. Ah Chong
From WikiPilipinas: The Hip 'n Free Philippine Encyclopedia
The classic case of United States vs. Ah Chong, G.R. No. L-5272 March 19, 1910, is still used as a textbook example of "mistake of fact" in criminal law.
Because of robberies happening at Fort McKinley, Ah Chong, a Chinaman, slept with a knife under his pillow. One night, he was awakened by someone trying to force open the door of his room. He thought that it was a robber so he stabbed the person who entered the room, who turned out to be his roommate.
Was Ah Chong liable for the death of his roommate?
Ah Chong was not held liable for the death of his roommate. The Supreme Court reversed the lower court’s conviction of homicide, saying that Ah Chong committed a mistake of fact. He would not have stabbed his roommate had he known the identity of the person who entered the room. If the person who opened the door had really been a robber instead of his roommate, he would not be criminally liable if he had stabbed that person in self-defense.