Couple that come to mind from many years ago:
Electrocuted myself with phantom. We've all done this before, but I had to pick a terrible time: in the wings, with an entire wind band of 12-18 year olds on the stage waiting for a soloist to finish. Changing a mic over, in a hurry. The cable had one of the switchcraft-type XLR connectors which was missing the grub screw that holds the cover on. Yanked the cable, cover comes off, hand hits the terminals... I'm sure the audience wondered what the band found so funny.
Lesson: If a cable is broken, fix it or don't use it.
Another red-faced moment: Was providing a couple of speakers mics for my high school's prizegiving. Forgot to change the battery in the radio handheld before the show. I'm being given a prize during the show, and the stage manager is so super organised he has everyone seated in the order that the prizes are being given out. So there I am, stuck in the middle of a sea of people, tightly packed so there's no way I'm getting out with anything remotely resembling subtlety. Half way through the show, of course, the battery in the radio decides to give up. But it's not going to go quietly, oh no. The dying mic lets out this horrendous noise, almost like a deflating balloon, to announce it's departure. And I'm stuck there, unable to do anything about it. Wanted to hide under my chair after that one. The worst part was, the award I was begin given that night was to thank me for my years of running and maintaining the school PA equipment.
Lesson: Fresh batteries before every gig. Always.
Oh, or the time the stage manager decided to 'help' me during derigging by moving a digital piano on his own. He put it on a flatbed trolley and attempted to wheel it out the hall by himself. In the process of trying to open a door for himself, he knocks it, and because this flatbed isn't really big enough for it, it tips over. I'm on stage coiling cables at this point, so I'm left watching helplessly from the other end of the hall as a grand+ worth of digital piano topples over and is damaged beyond repair. I still feel guilty about that one, it wasn't really my fault but I really should have spotted what the idiot was doing and told him to stop. It took a year to get that piano replaced.
Lesson: Make sure you keep an eye on the 'help'.