Here are my two worst moments from games I've been at. both came at what as then an annul pro-am basketball tournament, but different years.
(1) A guy went up for a dunk and his timing was off. He ended up falling to the floor and breaking ribs. Game was suspended until he was carried away on a stretcher.
(2) In a game involving my college and a team made of local pros, one of the guys from my alma mater's team took a hard elbow to the face and was bleeding profusely. The sight of that much blood almost made me pass out at the scorers table, but somehow I kept enough consciousness not to miss any action.