There is a very real, very disturbing, and very rare medical condition called "Alien Hand Syndrome" (AHS). An individual with this neurological disorder has full sensation in the rogue hand, but is unable to control its movements, and does not feel that it is a part of their body. The hand becomes personified, as if it has a will of its own, and its owner will usually deny ownership of the limb.
Though AHS was first identified in 1908, it was not clearly defined until 1972. Depending on the cause of the injury, the movements may be random or purposeful, and may effect the dominant or non-dominant hand. The symptoms are brought on by an injury to the brain, such as head trauma, stroke, tumor, or infection, but it can also be a side effect of a certain kind of brain surgery where the patient has the two lobes of the brain separated to relieve severe epilepsy.
As a side effect of brain surgery, or an injury to the corpus callosum (the area of the brain which connects the two halves of the brain), the movements are usually grasping motions in the non-dominant hand. When caused by head trauma, similar grasping and groping motions will often involuntarily occur in the dominant hand.
When the condition is brought about by a brain tumor, aneurysm, or a stroke, the alien hand may also engage in complex purposeful behavior, such as compulsive manipulation of tools, undoing buttons, or tearing clothes. Sometimes the sufferer is completely unaware of what the hand is doing until it is brought to his or her attention, or until they happen to see it themselves.
There is currently no treatment for Alien Hand Syndrome, but the symptoms can often be relieved by giving the rogue hand an object to manipulate, to keep it occupied.