The last time I went riding on a local hard surface trail with an older Specialized Hard Rock I've been putting together, I had a rear derailleur bust off.
The derailleur snapped where it mounts to the frame. I was able to disassemble it to get it off the chain, however, as shown in the photo below, the chain was twisted (unrepairable) during the mechanical failure.
I figured I could slip the chain onto a middle chain ring and middle rear cog and limp it back to the trail head, however, with the kink in the chain, this was impossible. The chain kept shifting itself from one cog to the next until it got hung up in the spokes. If I had a chain tool with me, I could have broken the chain to remove the twisted portion... a chain tool was about the only tool I didn't have in my kit that day.
Yesterday, we went riding on the same trail, but I was on the hybrid Schwinn I was working on. The problems I experienced were generally minor: brake cable and pad adjustment. I did almost lose it when the handle bars twisted (the stem wasn't tightened enough!) but I was riding at a low speed and recovered but it could have been bad.
Lauren riding her bike.
The repair that took the longest was Rachel's 1-speed coaster brake bike - the chain was thrown off the rear cog. To make matters worse, I didn't have a wrench large enough to get her rear wheel loose so I could get just enough room to slip the chain back over the rear cog. I did have a chain tool with me, so I was able to get the chain put back on, but I had to split the chain and fix it that way. Of course, I did not have any gloves with me or hand cleaner and working on a chain was messy... so that experience kinda was not much fun. Luckily, my wife was there to assist in getting the chain back together.
Rachel back in-action.
Lessons learned: 1) bring a pair of rubber gloves in the tool kit 2) find an adequate adjustable wrench that's still small enough to fit the tool kit 3) check to make sure bolts are torqued down - espcially on "project bikes"