Preparing for the eventuality that "STUFF" happens

As a photographer, you have to have redundant equipment so that when (notice I didn't say "if"?) something goes wrong, equipment fails, or you simply forgot to pack everything into your equipment bag, you won't have to run home and make the client/ subject wait. As well as seeming unprofessional, it wastes everyones time (and perhaps good light). The items that seem to have the most problems are flashes, and the accompanying cables, hot shoe adapters, etc...that go along with lighting the scene. I have known photographers who have forgotten to bring memory cards (me!) as well as the camera to a shoot. We're still human, after all...
So, having frightened you with the "what ifs", now to move on the what do I do to make sure this doesn't happen to ME! First of all, don't ever assume the batteries are charged, charge them every time. Have an extra set of batteries, in case the shoot is so magically delicious, you enter into a shooting frenzy, and use up a set. Have an extra COMPLETE set of cables, hotshoes, and recievers for wireless triggers, along with a 15 foot cable in case your wireless system goes on the fritz during a shoot. One light is still better off camera than having to use on camera flash.
Remember, it's not a question of "if", it's a question of "when".