I've broken a strap. Luckily the guitarist had a spare, so I spent the night playing the bass under my chin. I've carried a spare strap ever since.
I carry an accessories bag which has a spare for most things. Strings, gaffa, batteries, strap, pens, ear plugs, fuses and 2 full sets of leads. I even have several picks, which I don't use but I did lend them to the guitarist when he forgot his pick tin.
I also have a set of everything for home use. I never take any gig stuff out of the bags or cases. I've left stuff at home in the past!!