Alien Hand Syndrome
This one has been mimicked in movies and TV, but it's actually totally creepy. So one of your hands pretty much acts independently all on its own, which can often be very dangerous.
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Myostatin-Related Muscle Hypertrophy
Pretty much a body builder's dream. Basically you have reduced fat and a increased muscle size. So you're big and strong, and there are no real known medical problems associated with the disease.