Chain beaker. I put a 53-tooth chainring on my 90's MTB to get the top speed up. The ring just clears the chain stay but doesn't leave room for the chain if it falls off, so the chain will get caught and sometimes a link will get pinched or mangled beyond repair. I now also carry a masterlink pliers for the same reason.

I've also had spokes break causing wheels to go out of true enough to rub the frame.

Because my new commute is only 6 miles (vs the old 9-miles) and I'm now in my 60s, when I ride my rackless road bikes I've started to use a smaller, lighter backpack without all the extra tools. When I ride the bikes with rear racks I usually bring the whole kit and caboodle.