|Charles The Bald|
In 864 Charles apparently intended to play a joke on one Albuin during a hunt, ambushing him and stealing his horse. Albuin believed the assault to be genuine, and hit Charles on the head with his sword. Brain damage due to the blow left Charles mentally incapacitated. He never recovered and died 2 years later on September 29, 866.
Sources: J. Bradbury's The Capetians (Kings of France 987-1328) & wikipedia.