I had an arrow crab for a while and he always used to sort of unhinge his shell in his anal region (I have no idea what he was doing), but he died eventually. When he was at death's door he was sitting on the ground with that anal part of his shell open and the snail crawled right in and started ripping him apart. I've never seen them do anything else undesirable, though.
92-gallon corner tank, 100lbs of LR
, 140lbs of sand, 250watt 10,000K MH
, 110watts of actinic PCs, Mag 7 return, custom refugium, AquaC EV180 w/ Mag 5
Female lyretail anthias, eyelash blenny, tomato clown, saddleback clown, firefish goby, 2 sand-sifting stars