I've been a gigging bass player for almost 20 years. Before yesterday, I'd never had a piece of gear fail on me at a show. Never even broken a string, honestly. I typically only bring one bass with me, and trust that things will work as they should. Never again! My bass started crackling at my gig yesterday, then completely stopped working. Fortunately, it was at the end of the set, and there was a piano at the venue so we finished up with some songs I play keys on. Still, it was very embarrassing and I will be bringing a backup bass to every gig I do from now on! Also a bag with extra cables, strings, and my DI/preamp in case something unexpected happens. You gotta be prepared for anything, as I learned the hard way yesterday!
First time I had a real problem with gear was 1971 when my 1966 Fender Bassman amp started making this moaning, buzzing sound. I would smack the top of the amp head and it would go away for a while. I found out later that was just a tube that had gone microphonic and needed to be replaced. It was not enough amp for what I was doing at the time anyhow, so I traded it in for a Standel MCIIB/SAM30B piggy-back amp which never missed a second and was more than enough for anywhere right up until it was stolen from my house.
Other than that, I had an amp quit on me mid-song last year. Actually all it did was go into protection because it was clipping. It's a 225-watt amp and I was pushing it too hard into an Eden EX112, which will handle 300-watts. I think I may actually have a problem with the gain section in it because I couldn't get the gain-set light to come on. Will get it to the tech next month.
Anyhow, I made some EQ adjustments, turned up the gain and turned the master down, turned on the compressor, and everything was fine. In fact, that is the amp, I take with me now as a backup. It weighs all of 4.7-lbs. and will drive my DNS-210 cab just fine, My DNS-210 is a lot more sensitive than that Eden EX-112 and that TN226 is what I use to drive it at rehearsals (DNS-210 sounds really good with it).
Other than that, I carry extra strings, an extra wireless, a WTDI (DI with built-in Eden preamp and compressor) in case an amp dies or I am forced to use a backline amp, extra mic cord, instrument cord, extra SM58 (use a Sennheiser E-35 as my main mic), extra speaker cords both 1/4" and speakon, and extra 9-volt for my back-up wireless. I think that's about it. I still can't bring myself to carry my 5-string with me to every gig since I'm not using it, but I haven't had any problems with my bass since I bought it in 1985 and it's a passive bass anyhow (MIA Fender Jazz), so I'm not really worried about it.
I have used my WTDI a couple of times instead of my amp. Once when we were required to run a DI to the FOH and not use our amps at all. Another time when I showed up expecting to use the backline amp I'd been using for almost 2 years only to find that it was no longer there. Plugged my WTDI into the PA system and away I went. Also used it in a recording session at a Seattle studio where I had to share a room with the drummer and they were worried about bleed over...worked great!