I have been very fortunate that the worse injury that I have recieved in sword training has been a small cut from a plasitc Cold Steal bokken just the other day.
The worst injury that I ever dished out was on my roommates girlfriend. We were in class and it was her birthday. To celebrate, my sensei had the class fight her with pvc pipe wraped in foam. When I was sparring with her, I got too close and accidentally hit her with that butt of the handle (which was not covered). That dislocated her finger, and she had to go the emergency room.
It was nothing serious, and we laugh about it now. She really freaks out doctors now because she can tell them exactly when she got her tetanus shot.