When I was at the "Northrend Testing Area" today, I went to the point where the four Naxx wings are located (above WP)
Then I wondered what would happen if I get into the instance without using the portal but then realized that I AM still in the instance, because I got there using the glitch in Sapph's room. But I still went in and realized that Kel Thuzad doesnt' 'react' if you kill his beloved cat Mr.Bigglesworth (there's just no yell) AND after it I ran out trough the instance portal, re-entered the instance and the cat was there again. After killing it, there was the emote. So. My theory is that you don't get saved to the instance if you kill a boss there, just like the cat :s
Remember that you don't reset the instance if you run in and out (if you don't change the group) so that's not the reason why the cat respawned. If you do it with normal naxxramas, the cat remains dead.
I will post a vid how to get there, maybe you can try to kill a boss there, run in and out and look if it's still there?
everything could be wrong tho, but I can't motivate anyone to do it
epic song incoming btw