Well, fortunately I've never been involved with any real disasters. However, about ten years ago I had one rehearsal with an orchestra of the Saint-Saens 3, on a 57-rank Austin at the university here in town (which I rarely play). I got to the stage, powered up the console, and of course an earsplitting cipher burst forth. It really got the conductor's attention. It wouldn't go away, so I convinced the stage crew to let me climb up into the pipe chamber, high on the side wall of the auditorium, and after about five minutes of a C# going straight through my brain I found the pipe and pulled it. The following night, I powered up the console and checked everything well before the performance!
My most memorable performances, where everything just completely clicked and I could feel the audience hanging on every note, were (1) back in my grad school days playing the Allegro from the Widor sixth and (2) just a few years ago playing BWV 542...they were actually cheering at the end of the fugue, and it's in those moments of musical magic (whether I'm the performer or not) that I find God, whatever form he takes for you, to be most powerfully present.