Funny! Just today I had a similar experience on my warlock in End Time; I had actually accidentally queued for it (I had an epic drop from ZG in my bags when I queued), so I was around 350 ilevel. I get a group that starts by being douchey to begin with ("Pull now or I leave")... so we get to Sylvanas... and in the first 5 seconds the feral dps dies to purple fire. We continue the fight, but the dps wasn't enough to kill the closing circle mobs (never seen this happen before on my other characters, so I don't know if its ONLY my fault).
Of course, I start getting abuse as I was doing around 10k (compared to their 15k). Admittedly low, but hey, I didn't fail on easy mechanics.
So meh. I got rid of that pesky purple and am now back to normal Cata heroics :P

With roles reversed I had a hunter in my rogue's group doing 5k dps in Hour of Twilight, but the group quickly realised that he needed advice more than a kick so we carried him through. No wipes, no problem, and he actually got some gear in the end :).