Alien hand syndrome is an extremely rare neurological disorder, that causes involuntarily hand movements, without the person being aware of its actions. The afflicted person may sometimes reach for objects without wanting to do so, even to the point of having to use the healthy hand to restrain the alien hand.
Being diagnosed in 1909, Alien Hand Syndrome is also known as an anarchic hand. Since then, there have only been 40 to 50 recorded cases. It's being believed that other instances may have been misdiagnosed as part of an existing mental health disorder.