I've pretty much learned to bring a backup for everything that can possibly fail. guitar, strings, cables, pedals, amps, tubes, fuses... everything!
Oh, and my big one is this: Accepting ANY gig without getting all the info ahead of time!
So I had these guys in a local band hire me to play a show. They said they got a spot opening for a big act at a very large outdoor venue (like 10,000 seat venue) and their guitar player was out of commision. So I agreed to play. I was pumped, I saw some of my first big concerts at this place. I remember seeing Metallica, Van Halen and Skynyrd on that stage and was dreaming about how cool it would be to play on the same stage. Turns out that "opening" meant playing on the back of a truck in the parking lot before the show... ugh