One of my favorite films is Dr. Strangelove. In the film, the character Dr. Strangelove is a comically dark character who has lost control of his arm. Occasionally, while in the War Room, he puts his arm up in the air as if to say, "Hile Hitler," and knocks it down with the other arm. This is a subtle reminder to the fact that the US enlisted the help of Nazi scientists to assist with the building of the Hydrogen bomb.
What he suffers from is called Alien Hand Syndrome aka Dr. Strangelove syndrome. It is an extremely rare neurological disorder where the victim has lost control of one of their arms. Sometimes it is so bad that they don't even notice until it is pointed out to them. It happens when the connection between the brain's two hemisphere is severed. Sometimes it is caused by surgery for extreme cases of epilepsy.