Originally Posted by jimmuller

Q (from the pic): Where are the outer plates, i.e. the missing link? It looks like a plate pulled out from its rivet, starting the rest of the process. Once one end of an outer plate was free it could have induced the other end to come free soon enough, and then both that plate and the opposite side with rivets attached came out. No telling how long you pedaled with just one side plate on that link. As long as you weren't mashing too hard and the periodic entry into the RD cage pushed it back in again you could have gone on for a while that way. But with no stabilization from the opposite rivet any serious tress could have pulled the remaining rivet out from its remaining plate.

So an episode like this could have been precipitated by mashing harder than normal, but its real cause may have been a poorly fitting rivet.