I have been reading the recent thread "Emergency Wedding" and thought it would be a good idea to talk about the incidence/likelihood of equipment failure at paid gigs. I know a number of people advised the OP to have a backup body available. It is excellent advice that I hope he will take to heart.
I take excellent care of my gear, which is usually purchased new. However, I still experienced two major equipment failures in 2012.
The first occurred at a boudoir shoot, when the Skyport trigger failed. I replaced the battery, no help. I attempted to use the studio owner's Pocket Wizard, no luck. Finally, in desperation, I pulled out my 7D, engaged the built-in flash and used that to trigger the strobes. The camera was tethered to my laptop and the client (a photographer herself) was happy with the results. Now this seems to be a shoot that could be rescheduled, but there would be expenses incurred which would move this from a modestly profitable shoot to a possible loss. Specific expenses would include the possibility of having to pay studio rental again, plus the client had paid for her hair and makeup to be professionally prepared. It turned out that the Skyport trigger had indeed failed, and it was promptly replaced by Elinchrom under their two year warranty.
The second failure occurred at a wedding, using a two day old 5D2. No problem during pre-ceremony, ceremony, formals. At the reception, I noticed that I was sometimes getting correctly exposed images but I was also getting images that were totally blown out to white with no detail. I had no way of knowing whether the problem was with the body or the flash, and there was no time to try to resolve an issue. I just packed it up and once again pulled out the 7d which I used the rest of the night. The bride never knew that anything had gone wrong, so the rest of the evening remained a time of relaxed celebration for them. Canon repaired the 5d2 immediately and it has never presented another problem.
I hope this post helps anyone considering taking on paid jobs without having back up gear. Have a great day.