I have 2...
1) Huge outdoor festival... 100 degrees on stage, so staying in tune was iffy to begin with... We all used backline which included a tuner none of us were used to, and it was calibrated differently for every one of us!
Trouble is, we didn't know it until the first chord of the first song! It was supposed to be a long slow build-up to an epic huge chord...
Build-up, anticipation... WHANNNGGG!!! Train wreck.

Sounded like a kindergarten music class.
2) Like a good (read: cheap!) active bass owner, I always unplug my bass between sets to preserve battery life...
Live TV gig, AM morning show... No sound check. Come 0730, plug in and go.
After the song is over, host talks to the band, builds us up for the next song in an hour. Host throws it to the "on location" guy. So far, so good...
So, per usual, I reach down and unplug my bass. Have you ever heard the sound of an electric instrument being unplugged from a live board? Imagine that, but on live television where the sound reverberates through the whole studio, as well as through the live feed. It sounded like 50 tons of TNT going off...
Ya know, I coulda remained anonymous if the host hadn't pointed it out on the air...