Well, this is kind of a vague and obscure dick moment, that i have actually had happen to me a couple of times, and it can even be argued whether or not it actually is dicketry (thats right i made up a word). So I run a resto druid who is all 397/403 geared right now and I try and run ulduar pretty often because I still have not gotten the raid achievement Glory of the Ulduar raider (10 man), and this is why: every single group that I head up has had the exact same problem with the exact same achievement, the Disarmed achievement, and everytime I try a different method to down it. And of course as always someone manages to screw it up by either not killing something fast enough when i say to switch or killing one of the arms too fast and screwing up the timing... idk.. maybe the WoW gods just hate me and don't want me to get the rusted proto-drake, or is all of the WoW universe trying to screw it up for me?
Gaiaticus= 87 Tauren Restoration/Guardian Druid (Alleria)
Shaianne= 85 Blood Elf Frost/Arcane Mage (Alleria)
Chihiru= 46 Pandaren Mistweaver Monk (Alleria)