Kalon at ThinkTank touches on a subject near and dear to my heart -- missing out on loot upgrades because you forget what you're supposed to want. While on an Ulduar-10 run, he passed on the Kologarn drop Mark of the Unyielding, which is close to being the best-in-slot neck for bear tanks, for one good reason (the other tank raids more than he does at the moment) and one bad one (he couldn't remember just how good the piece really is). Particularly galling, he notes, is that he recently wrote an excellent guide to tanking upgrades in Ulduar in which Mark of the Unyielding was singled out for especial commentary. Ouch.
For myself, I don't think anything quite approaches the sheer agony of having accidentally passed on Idol of Worship from Grand Widow Faerlina just because I wasn't paying attention at the time. I don't do much cat DPS, but losing the best-in-slot idol directly affecting the attack (Rip) that contributes the largest share of feral DPS is...again with the ouch. I still ask myself what was so amazingly enthralling about tanking Maexxna trash that night that I could have zoned out to that degree, and -- naturally! -- the idol never dropped again. While I guess it's possible that there are players out there who are practically perfect in every way and have never missed a drop, I have the sneaking suspicion that most of us still find a reason to kick ourselves.