The way Murphy's Law works at gigs is that if you need something you don't have it, but if you have something you don't need it. I find the second part easier to live with. A soldering iron, spare jack plugs and "third hand" are things which I always like to have - at least nearby in the car. I lost a pitch when I was busking on the Leidseplein in Amsterdam once when I broke a string in the dark and took out a bridge pin at the same time. I had strings of course, but no bridge pins. You live and learn...