Sindragosa-10 with about 2,100 health left. We finally killer her a couple of attempts later. It was still pretty soul crushing though, that's *one single melee swing* from death. We'd spent 3 evenings on her to that point. On the other hand, we went on to one-shot Putricide, having only killed him once before, which made up for it a little bit.
Back in TBC, we'd got as far as Aran or so in Kara when Moroes started sulking, and we spent several weeks unable to kill him. One of those attempts was a sub-500 health wipe. In another, the offtank had issues holding secondary threat. I was playing a Mage at the time. MT got gouged, Moroes ran over to me. He missed me 5 times in succession, and then ran back to the main tank, who promptly died. I survived that one \o/