Ten years ago I photographed my first wedding, located in a small farm town in northeast Indiana. My equipment list for the first few weddings included a Nikon d80, Nikkor 18-135mm kit lens, Nikon SB-600 speedlight, six Sandisk SD cards and a rental studio lighting kit. I carried no backup equipment.
After the first few weddings, I learned that I needed to have backup equipment with me at all times. During the few months where I wasn't fully prepared, I was very lucky to not have any equipment failures. I quickly purchased a spare Nikon d80 (never had to use it) and a new Nikon SB-600 speedlight. At some point over the years, my original speedlight died and my replacement took its place.
Now that I am 145 weddings into my wedding photography career, I am vigilant about carrying backup equipment. I carry multiple camera bodies, lenses, two-times as many memory cards as needed, speedlights, studio lights, batteries and even a spare pair of pants. I want to always be prepared! I even know where the closest camera store is in the event of an absolute emergency.