We all know things can go wrong and sometimes they might be due to circumstances out of our control or due to a simple mistake. I'm certain we also know how important it is to be prepared for those unfortunate but inevitable situations. Clients, however, may not be as understanding and this is why it's important to have measures in place to ensure continuity.
In a recent and helpful video, Tony Northrup discusses some of the options we can do to prevent issues and also make sure we can continue working. Continuity is the most important factor and this can't be stressed enough. If for example you're shooting a wedding or working for a company it's simply not good enough or acceptable to not be able to deliver due to your camera failing or because you failed to backup your files. This is a great reminder to us all because, no matter how long you've been working in the industry without any issues you never quite know whats around the corner. My favorite suggestion from Northrup is to have extra SD cards packed anywhere you can. SD cards are relatively cheap and if on the off chance you somehow forget storage cards for a shoot, you know you have backups available. I know early on in my career I've had a shoot that I've turned up to without an SD or CF card. Luckily I was able to borrow from a friend, however, it could have been much worse.
I highly recommend you check out the full video for all the suggestions and review your practices to prevent any issues.
Everything essential must be redundant. Three is two, two is one, one is none. And this is based on what you show in your portfolio. If you'e promised the client what you showed in your portfolio, then that equipment is essential for you. Including you...have friends you can call to pinch-hit for you if you can't make an appointment.