Many things can go wrong in the day, so we need to be well prepared. We must have a backup plan (in case of bad weather), charged batteries, blank memory cards, think about the routes and time to get to the places and get a full day itinerary so we know what will happen in the next moment. If possible, we attend the ceremony rehearsal where a large amount of information is gathered about possible positions to shoot from the lighting, the order of the ceremony, etc.
Most new wedding photographers skip hiring a “second shooter” to back them up on wedding days. If you can possibly make it happen financially, it is definitely worth the money to hire another photographer to work with on the wedding day. The photos will be better, you'll have a second set of gear in case of disaster, and you're extremely unlikely to miss the shot. (Thanks Gabrielle Walker-Jones)