We've all heard of Murphy's Law, "Anything that can go wrong, will go wrong". We all try to avoid this, especially in business since it is a direct reflection on us and our brand.  The first way to avoid the 'inevitable' is knowledge, the best way this is achieved is through experience, we learn through experiences, mistakes and learning from them.
Another way is being prepared, 'just in case' something happens, this is something I learned a long time ago. When going on location, I always carry backup, yes, it makes for a larger load but I know I'm prepared just in case, since I've been through several situations and learned from them. I always carry a backup camera, lens, batteries, backup flashes, cords, etc. that is, I have at least two of each just in case, say, a camera falls? This has actually happened a couple of times, as stressful as it is, I carry on with the session objective in mind and deal with the problem later, because I have a backup that will complete the job. One example was a wedding about 25 years ago when I was moving from the church to the reception and the camera fell off the tripod on to the concrete, when I picked it up, all i could hear was broken glass inside the camera (in the film days). I had another camera that did the job just as good and carried on. I once had a strobe start to smoke and over heat, I immediately unplugged it and replaced it with the backup.
Another way of being prepared is knowing the environment and situations of the location. As much as I communicate about the photography project, there are times were I'm not able to see the location until I arrive. On these occasions, I bring more than enough equipment to handle a variety of lighting scenarios, my theory is that it's better to have too much, than not enough
It's all about making sure the customer gets what they want and delivering the quality images they expect in a professional way. If you ever have a session where the person taking the photos shows up with only a camera and flash, be forewarned, not only are they not prepared, but they also don't have experience needed to provide the images you really want. Â
Shown below is a sample of the equipment taken to on location jobs, it always varies. Not shown are the light modifiers, tripod and stand.