Had an interesting problem recently with my 5d Mk III. Firstly couldn't get a Yongnuo YN-622C remote off the hotshoe. The screws holding the hotshoe onto the body loosened up meaning I couldn't get the locking pin up high enough to disengage the bloody thing. In the meantime I couldn't use the flash on it without getting a full power unmetered flash. PITA. Finally all of the screws loosened enough for the whole bloody thing to come off including the four screws. Re-attached it and tightened down the screws (minus the remote of course), and hey presto....all fixed! ETTL is back again.