At mile 45 of a 70 mile gravel race last year, hitting one of the descents with some speed, my rear wheel suddenly locks up while trying to pedal. I come to a stop, to find that the rear derailleur hanger bolt had backed out and wedged itself between the cassette lock ring and the spacer. I admit that it was the 1 bolt that I didn't do a pre-race check on.
Anyway, I tighten that back up and make it about 1/2 mile before my shifting goes to crap. My first thought is that the hanger is bent...but it's not, come to find out that the cassette lockring must have been loosened by the hanger bolt and then backed completely off in the ensuing half mile. Since a lock ring tool isn't something I'm going to carry in my seat bag, I "fixed" it with a slotted screwdriver and the application of a rock. It worked and I continued on to the finish.