So last week I killed an old XT rear mech. I say killed, it really just died of old age and started flopping about. It owed me nothing.
This weeks night ride I seem to have killed either a four week old SLX one or bent the hanger. Now I have not yet looked at the problem other than a cursory glance, but which is more likely from a stick/mech/wheel interface:
Bent mech hanger. Nothing I could see with the naked eye at the time.
Symptoms being a loss of indexing with not being able to get it sorted inside the socially allowable group ride timescale.
Is currently in the huffy corner of the garage awaiting a spare bit of time. My bet is on the most expensive permutation whatever that may be.