When you're out in the field, it can be a big problem if something you are relying on breaks. Trust me, I know.
One of the "interesting" design aspects of the Arca-Swiss quick release tripod clamp is that for whatever reason, if you unscrew the knob all the way, it will come off in your hand along with the spacer sleeve that sits behind it. Even knowing this, I fell victim to a lost knob when it managed to work its way loose while hiking on a trail. Fortunately, I was hiking back to the car at the time, but it was still not a fun experience. Ever since then, I carry a spare knob and sleeve with me as spare parts.
Many things can go wrong and it's best to be prepared. Along with the above, I also carry some duct tape, a piece of wire and a few other odds and ends. All together, these items don't take up very much space, and they're good insurance that helps lessen the possibility of a ruined trip from equipment problems.