You have to be prepared for all sorts of issues. Having to stop the show due to gear failure will not win you many friends. I don't play for money very often, but I usually have spare strings for the guitar, a battery if it needs one and a spare lead.
I've seen people break strings or have cable trouble. Also have a decent strap on your guitar as I've seen those fail too.
Here's a fail ;)
It can be an absolute nightmare, but as long as you are prepped for anything, you can make it work out.