I usually have 4 instruments on stage with me, 6 string fretted, 5 string fretless, 5 string fretted (alt tuning) and the Chapman Stick. it would be nice if I could just bring one, I'd still bring a back up though. last gig we played, just an open mic so I brought one bass, for 2 songs, but I was worried.
I've had my amp blow up on stage twice, (free smoke show!) once at the Rat in Boston, once on a blues booze cruise, it continued to work, but only at half power (thanks Gallien Krueger, I will never buy another of your products!, they sent me a new amp in my old chassis and that one blew up too xp)
I had a bassist ask to borrow my bass but he played 4 and I only had 5's with me, also there was no way I was going to let that particular bloke touch my bass. as it was the singer put the mic down his pants, much to our vocalist's chagrin as she had to use it after.
had a battery die too, sound-guy said to me "sounds like a blown speaker" right as we were going on, I freaked, but played the whole set on my fretless, including tunes with hammer-ons and chords which I'd never even tried to play without frets before, it all sounded great as it turned out, fortunately, I surprised myself. we failed to win that battle of the bands as we didn't bring in anyone in to buy beer.
and the moral of the story is be prepared for anything. bring band aids too.