My favorite was seeing our hunter disengage off the pillar in heroic LK fight and listening to him scream "Noooooooooo!!!!!" in vent. I just about pissed myself I was laughing so hard. I remember my rogue trying to finish off a Valk in that same fight and popping KS too late and following them off the platform, fallign to my death. Also, my rogue popping KS at the wrong time on the heroic guy who calls the bloodbeasts in ICC (right after lootship, cant remember his name) and causing a wipe. I know these arent hunter derps, but they're still good for a laugh.
OT - not a derp, but i used to laugh my ass off on lady deathwhisper (i think thats her name, the MC one) when I would get mc'd right as my cds came up and basically 1-shotting healers. All you'd hear in vent was: me - "shit, i didnt get my cds off before i was mc'd, someone cc me". healer - "fuck, stabby killed me again". RL - "Goddamn it stabby, stop killing our healers". o many lols to be had. Same thing when our ret (2nd on server to get shadowmourne) would get mc'd - "Oh shit, someone's gonna die"