Originally Posted by
lostarchitect
You may need a new chain/cassette, but you may just need to adjust the stops on the rear derailleur so it doesn't go past the cog.
Agree. Components *may* need to be replaced. But the chain coming off doesn't indicate that. I've never seen a chain do like that just because it's worn out. The problem is the chain-stop. Then maybe more too but there's no clear indication of that.