Have you heard of the condition known as alien hand syndrome?
It's a neurological disorder that causes people to lose control over their hands. The cause is usually some sort of brain damage. Injury to the fibers that connect the left and right halves of the brain can make your non-dominant hand make involuntary grasping motions. Damage to the frontal lobe can cause the same result in the dominant hand.
Other brain problems, like a stroke, brain tumor, or aneurysm, can make either hand do more complex things like button or undo button and take off clothes. The truly strange thing is that the person isn't even aware of what the hand is doing until he happens to notice.
People with AHS often think that the affected hand is possessed and don't recognize it as part of their body.