got plenty of headaches for playing too long. ;-)
plus eye strain, tired.
RSI or CTS are probably the most likely conditions arising from the modern mode of (electronic) play. Considering the traditional equipment only ... http://www.chess.com/forum/view/fun-with-chess/chess-injuries2
One trick that doesn't work well electronically is to have one or two surprisingly heavy (or light) pieces among the opponent's colour. You set up the board. If the piece is unusually light, the opponent may tumble (works better if you are both perched on stools); if the piece is heavy, he may get a slight cramp when he tries to lift the piece - or he may play an easier-to-lift piece. (p.s. I haven't checked whether this falls foul of FIDE rules.)
Has a chess player ever got injured like straining a muscle or anything?