Just Dont. have a secondhand camera/lens - ideally the same make and ideally model as your main camera. If you get really unlucky and have a malfunction with both (really unlikely) , look out for a guest with the best camera and use that (or get them to) until you get someone to bring you another camera. As a final final resort, use a good smart phone until emergency supplies arrive! Probably the worst time for this to happen is during the ceremony, if all other options are not possible, see if you can get a delay to get help. That is better than the alternative of not being able to photograph it.
If you have decided to take the risk and only use one camera, you won’t get any sympathy from me or more importantly from the couple and their families!
This is why it is so important to have a good network of fellow photographers. I have delivered a lens to a fellow wedding photographer once when hers was dropped during a wedding (drops happen and will happen to you eventually trust me!). I wasn’t doing a wedding that day - so I was more than happy to help.
Finally, make sure your wedding photography contract covers you for equipment failure. It can happen, so make sure you are protected.
Don't do wedding photography without it. I use Photoshield.
Again, don’t photograph weddings professional without a wedding photography agreement with the client co-signed.
Take some shots of the sky using aperture priority at around F16. If you see spots either take care of this yourself if you really know what you are doing, or arrange to get your sensors cleaned by a reputable specialist. sometimes cameras have a Where I am based (South Coast) I use Cameracal, Park Cameras or Fixations. If your camera has a sensor cleaning option on the menu, I often use this after taking the camera out of the bag in case any dust has been disturbed during the journey. I may use this again during the wedding (if I remember!). In real emergency situations where on the day you know you have a prominent spot and can't get rid of it , take extra shots of important moments at very slightly different compositions so you can clone as required. In reality don't stress too much; spots mostly only show at narrow apertures and can usually be cloned out. Some cameras have an option to avoid disc spots, but I don't use that much myself.
If you notice oil smears on your sensor, get this looked at by a professional or the supplier.
You may have forgotten that your last shot at the previous wedding had you lying in mud trying to get that last twilight hero shot!
Take all the list of equipment etc as above.
Aim to arrive an hour early for medium distances , more for longer, but always before. Traffic, accidents, tyre bursts etc are all things that could make you late. Never be late. If you can, avoid Motorways, as a sudden hold up can leave you stranded. Have a Satnav with live traffic warnings, listen to local radio. If you have to use Mortorways, give yourself much extra time, just in case.