Alien Hand Syndrome happens when there is unusual damage to the brain, sometimes following surgery, stroke, or an infection. Sufferers of the neurological condition usually retain feeling of the hand, but not the movement. Many say that their hand appears to "have a mind of its own", and it can "perform complex acts such as undoing buttons, removing clothing, and manipulating tools"!
Much like in movies such as Dr. Strangelove, the afflicted typically has to use his or her other hand to keep the "alien hand" at bay.
The human brain is unbelievable!