The worst I've ever had is blisters on my thumbs from playing Street Fighter 2, back in the day...
Oh, and to keep Clive happy, I know one kid who had a fit a few months ago because he played on his GameCube for several days straight. He had to go to see the doctor and they decided he had a mild form of epilepsy. That said, he now has to wear sunglasses whenever he goes outside because bright lights may trigger another fit. I presume he was eplieptic all along but it took several days worth of gaming to kick it off. I feel a bit sorry for him, but that's literally all he ever does. Seriously. He has no hobbies other than playing video games all day long... :\