The Alien Hand Syndrome is a disorder in which the hand of the sufferer seems to have a mind of its own

Alien hand syndrome (anarchic hand or Dr. Strangelove syndrome) is an unusual neurological disorder in which one of the sufferer's hands seems to take on a mind of its own. AHS is best documented in cases where a person has had the two hemispheres of their brain surgically separated, a procedure sometimes used to relieve the symptoms of extreme cases of epilepsy. It also occurs in some cases after other brain surgery, strokes, or infections.

An alien hand sufferer can feel normal sensation in the hand, but believes that the hand, while still being a part of their body, behaves in a manner that is totally distinct from the sufferer's normal behavior. They lose the 'sense of agency' associated with the purposeful movement of the limb while retaining a sense of 'ownership' of the limb. They feel that they have no control over the movements of the 'alien' hand, but that, instead, the hand has the capability of acting autonomously--i.e. independent of their voluntary control. The hand effectively has 'a will of its own.' Alien hands can perform complex acts such as undoing buttons, removing clothing, and manipulating tools. Alien behavior can be distinguished from reflexive behavior in that the former is flexibly purposive while the latter is obligatory. Sometimes the sufferer will not be aware of what the alien hand is doing until it is brought to his or her attention, or until the hand does something that draws their attention to its behavior

http://en.wikipedia.org/wiki/Alien_hand_syndrome