As for worst mishap on stage, hmm...
Well, the first incident that comes to mind happened the during the previews of
Lestat. The opening of the show was just not working, and they tried a couple of options. The one option that seemed to work the best during rehearsals was finally instituted one night. Since there were a lot of lighting and automation cues, they had set up a click-track so that everything would coordinate. Well...
We're all sitting there waiting for the lights to go down, and to see if the new opening worked. The lights go down, we hear the pre-show announcement, and the conductor's cue light goes off - which is the signal for the start of the show. As soon as the first downbeat was given... We start hearing the click-track counting down in our headphones.. "1 - 2 - 3 - 4"... I looked over at the other keyboard player who was also on headphones, and I could that we were both wondering if we should start again... Or if they were going to go ahead and stop the show a few measures in just to make sure we got the opening correct... In the meantime, the conductor is conducting up a storm... Then we notice him look up at the stage... Then we hear the click track change tempe... Then we all notice the conductor just looking around wondering what to do... Realizing that he had jumped the gun... So, we finish the Opening Sequence, and sure enough, the cue light starts blinking on the podium - which is the signal to pick up the stage management phone. And, even thought we couldn't hear what was being said, just following the looks on Brad's face for those couple of moments was, well, for lack of a better word, priceless.
*And I believe he bought the drinks for the band that night.

**And we never did try that version of the opening in front of an audience.
***And the following night, the other keyboard player and I who were on closed circuit cameras up on the seventh floor made big signs that we held up to the conductor's camera: "1 - 2 - 3 - 4".