During its first ride, my brand new XT Shadow Rear mech committed suicide. I had only gone about 20 miles on some fairly rough terrain and, as usual, was the furthest away from home!
I had treated my main ride to a new chainset - SRAM 991 chain, XT cassette, new raceface rings and an XT Shadow Rear Mech. When i was setting up the new mech, I was a little disturbed that it ran so close to the cassette, but spent a lot of time making sure that the B screw was undone enough to make sure that there was sufficient clearance.
I made a naughty gear change - a 42T front to a 34T rear whilst i was on a bit of muddy singletrack and crunch - the mech caught the cassette and twisted back into the spokes and broke the chain!
I bought a new XT shadow and was surprised to see that they have changed the design. The little jockey wheel guard sticks out a long way and is a terrible design. Because my bike was a single pivot bike (much like a K2 or Proflex) the chain shortens when the suspension is compressed - this compression with tight clearance caused it to mash up.
The new XT Shadow mech does not have this problem... So if you are upgrading, make sure you have the new type.
You can clearly see that the old XT Shadow mech has scratch marks where the cassette hit. The B screw was set right leaving massive clearance - much more than the 5mm to 6mm the instructions suggested.