Edit: Broken recount is broken. So let’s address the rest of the fail, because that’s still perfectly valid. :P
If you don’t know an instance, speak up. Better you ask than have your healer screaming obscenities at you IRL. I mean sure, I’ll wonder inwardly how the hell you don’t know NEXUS, but that’s better than calling you a douchenozzle becuase you’re making my life insanely difficult, right? I’ve had several cases of DPS wiping groups because they didn’t know fights already (though it makes more sense in the new heroics). Bwuh? ASK, PEOPLE. At least for the new heroics, I stop and ask if people know the fights first. If you don’t answer and then wipe us with something stupid, I’m going to be pissy.
If a boss has spell reflect, it behooves you to notice your own dots stacking up on you and STOP. PLEASE FOR THE LOVE OF GOD STOP. If you keep taking a ton of damage, your healer is screaming JUMP at you in party, and you notice a stacking debuff? Yeah. Do it. Please.
Speaking of heroic fails, my lovely tank healing, high crit, low haste build? Yeah, it fucking sucks for heroics. Oh the normal ones are fine, but if you get in the new ones with even a semi-derp derp group? Yeah, kiss your sanity goodbye. You can’t have low DPS in there. You just can’t. I know this because I tried. And I couldn’t just group kick because they were guildies. Bwuh.