The most recent onstage accident I witnessed was in the CLO production of OKLAHOMA! earlier this year: Jud and Curly are in the smokehouse about to shoot a knot in the woodwork, when the gun won't go off. They passed it back it forth a couple of times before Shirley Jones (as Aunt Eller) came onstage shouting, "I thought I heard a couple of guys out here trying to shoot a gun."
Personally, in the closing performance of FUNNY/FORUM, the incredibly hunky Miles Gloriosus, who quite unfortunately had a not-so-secret crush on me, decided that since I had my own date in the house, he would punish me during the funeral by punching me (as Hysterium). The punch landed, pardon the expression, in the nether regions - and being a beautiful dead virgin at the time, there was absolutely NOTHING I could do to retaliate. . .ooooh, the pain. ooooh, the anger. After exiting, I had to be physically restrained from coming in contact with Miles and my stuff was quickly moved from the men's communal dressing room, to the ladies, so that I would Mr. Miles would not have to face my wrath.
When doing EQUUS, the second night of the run, the power went out one third of the way into the first act -well, just as the beach scene with Alan's parents and the horseman was about to begin. After waiting for a powerful return, we polled the audience, who chose to HEAR the rest of the show as if it were being done on radio with only candlelight.
While in THE MISANTHROPE, updated to 1980s Hollywood, I had a quick change (30 seconds) into short-shorts and tank top, more often than not, the first time that I would sit in the next scene, parts of my anatomy not meant to be seen. . .well, this is a family site. . .you're all smart enough to fill in the blanks.