I just picked up a Mavic 851 rear derailleur that looks exactly like the one pictured at velobase:

Mavic 851 rear derailleur

The problem is that, for whatever reason, the inner plate of the derailleur cage (closest to the spokes) is really worn through from the chain rubbing. Even part of the bolt that allows you to slide the cage adjustment up or down (making it a long cage, medium cage, whatever you want) is a little worn.

This derailleur doesn't have the original Mavic pulleys. Instead having some goofy Bullseye red anodized metal pulleys:

Bullseye anodized pulleys

Why would the chain hop off the pulley and almost completely wear through the derailleur cage? Has anyone ever heard of this happening before?

Does the Mavic 851 use washers on the derailleur pulleys? The derailleur had eight little metal washers. Two each went on either side of the Bullseye pulleys. I disassembled the cage (aren't Mavic components great?) and I'm just shocked at the fact that the bottom of the cage is almost completely worn through from chain rubbing.

What caused this? In getting this derailleur ready for use on a bike again, should I use those washers? Are they part of a stock Mavic derailleur?

Can you replace the sealed bearings in a Bullseye sealed bearing pulley? Has anyone tried using a 'floating' pulley in one of these Mavic derailleurs to improve shifting performance?