My worst was letting an Oompah band convince me to play a Cajun tune on fiddle with them. I thought they were a little shaky on Jolie Blonde and when it came time for the vocals (Long drawn out note in C) the band decides G is a better chord to be in. I've never been so red in the face in front of 500 people.
The worst one I've heard of happened to a guy I played with. This musician, (who will go unamed for obvious reasons) plays with a major local cajun band and had some stomach trouble before going on stage. He realizes hes gotta take a crap midway through a long medley and with no options, lets one loose while hes backed up against the bass amp. Realized he was wearing baggy shorts and no underwear a little too late. Made me feel better about my nightmares on stage.

Okay, I've got my own experience with a bad sound guy. I was playing in Paris with the Magnolia Sisters and the sound guy was SMASHED. I mean a few bottles under, at least. First, he almost put the monitor down on Mom's fiddle. Second, when the battery died in my preamp, instead of changing it, he unplugged it, plugged me straight through the amp, and my very wonderful, rich-sounding Martin wound up sounding like a mandolin someone pulled out of a dumpster. During the break between the second and third sets, we actually WITNESSED him fall off of his chair, onto his back, and smash a full bottle of wine. YAY!