So I had my bike up in the workstand cleaning the gunk after the very, very wet ride I had earlier. In the lower gears, the chain jumped off the lower jockey wheel, jamming between the jockey and cage.
Not being quite sure what was causing it I started to look into it. I initially didn’t think it could be Todo with the wet ride so I looked at the jockey, not worn, so cleaned it off and ensured it still spun freely. Put it back on and the same thing happened. Ok, perhaps the derailleur cage is bent. Nope, that was fine, so check the hangar, that’s perfectly straight.
Could it be the chain? Time to break out the chain cleaner. A good scrubbing of the chain, and bingo, no skipping off the jockey. So all I can think of is that some filth had worked into a chain link causing it to be skipping off the jockey.