My first time in Zul'Farrak, during the Pyramid event (or as I like to call it now, the ZF Ziggurat Zerg) the entire party died except me. I was on my rogue, so in order to help keep the NPCs alive I had to frantically run around bandaging them between waves, and keep myself alive with bandages and pots, all while the party slowly corpse-ran back to the instance and then ran back to the pyramid. By the time they got back I and the NPCs were making our way back down the stairs and were nearly dead, but they came back just in time to keep us alive from the remaining trolls. Probably the most hectic/frantic experience I've had in the game.
The "one man defense" of Alterac Valley. On my BE pally, I was all alone on defense against the Alliance offense, who had already taken the Relief Hut and both towers, and were standing in front of the keep, probably planning who was going to pull/tank the Warmasters. I mana tapped a random hunter to aggro his pet (this technique will work with just about any ranged offensive ability, and will also work with warlocks, of course). I ran back inside Frostwolf Keep, and sure enough the hunter wasn't paying attention, so I was able to get the pet in range to aggro the Warmasters. They kill the pet, and the hunter who owned the pet gets their aggro. They all run outside after him, aggro the entire Alliance offensive, and slaughter everyone. I subsequently continued to use this technique in future BGs to great effect.