What is going on with my derailleur? I was on a ride a few months ago and while shifting, my chain came out of my derailleur. So this is a little confusing. Let me explain. The chain was resting on the outside of the bottom part of the rear derailleur cage. There is a little silver cylinder behind the lower jockey wheel that touches but doesn't actually connect the two sides of the cage. Somehow my chain go pushed to the outside of that. When it happened it felt like the chain had gotten shifted off and my cranks jammed up. I stopped pedaling as quick as I could.
Anyways, to get home I had to take out the bolt that secures the bottom jockey wheel in the cage, and open the cage up to get the chain back on the correct side of the silver cylinder. I put the jockey wheel back in with the chain in place and limped home.
I took the bike to the LBS and explained the problem. They didn't know what might have caused it. They checked the hanger, and it was a little out of alignment, but they said it wasn't that bad and fixed it. I also asked them to check that the derailleur itself wasn't bent, but who knows if they actually did that.
Flash forward to yesterday and the same thing happened. I fixed it the same way and limped home again. I've never seen this before and definitely don't want it to happen when I am sprinting.
A few things:

My derailleur is an 11 speed Dura-Ace.
Chain is not worn/bent/no frozen links
Cassette is near new
I was shifting both times, but not putting a lot of power down.
Hanger should have been OK.
The bike has less than 1,000 miles on it
I didn't notice any wear on the bolt or threads and the bolt was tight as could be with locktite in the threads both times

Any idea what might be going on/how to fix it?
I see no way for the pictured setup to occur while riding.


The silver bushing is bolted or rivetted to the cage plates
Cage plates are contiguous plates of steel with no gaps
Lower jockey wheel is also bolted to chain plates.


These three items form a box through which the chain runs.  No amount of folding or flipping the chain would get it out of that box.

The only options I can think of are


Someone misassembled the chain in the first place - Plausible because the chain is low tension at that point, and you may not notice the clattering.  I've personally ridden with a group ride up a gravel track and someone else only noticed the same problem halfway up (hi Brendon!)
Notice the scarring on that bushing - the chain has been running on it for a while.
Someone has fiddled with your chain while the bike is out of your sight. Unlikely because its a dirty business messing with chains and that's a long way to go for a joke


If your chain has a quicklink then fixing is easy and can be done roadsize with pliers, or some people can split them by hand.



Opinion  If the bushing is only attached on one side. then its not doing you a lot of good.  Consider removing it completely and see how things go.  Store the small part, in case it gets worse and you need to refit it later.