I've had two minor gig mishaps:
1) I had a strap break. I had a spare on a different bass and switched it over.
2) I had a wireless unit and the way the cable from the bass to the transmitter on my strap was hooked up, it was pulling out of the wireless unbeknownst to me. I finally figured out and the show went on.
Not me, but my guitarist in a previous band. As we were setting up for a show, the house PA and mains were set just in front of the sate. The side the guitarist was on had a bar next to it and as one of the patrons went by, they nudged the front main tripod and it want tumbling over (a 15"), narrowly missing my guitar player as he was crouched down adjusting his pedals, but sending his tube amp head off the top of his cab and to the door while also knocking his guitar to the ground. The tube amp casing was crooked and his guitar has a massive gouge in it. His amp still worked and we managed to play the three set show, but it crapped out on the last tune.
It turns out that whoever setup the PA didn't extend the tripod legs for the speakers wide enough and just a slight touch sent the main to ground. It was an accident waiting to happen and it did. My guitarist was rattled for the whole show. He was inches away from a potential significant injury.
The bar paid for his amp replacement as guitar repair.