Killing Vulpes.
On my first playthrough I wandered into Nipton wearing NCR armor without a clear idea of what was going on in town. I round the corner and instead of a pretty speech I find a dozen legionnaires charging me, and one of them is wearing a weird helmet and oh god why does he have a chainsword?!
Ducked into a building to restore health, and it just happened to be the one with the survivor in it. Reminded of my reputation as the Powder Ganger Grim Reaper and now having a clear idea of the evil I was facing, I charged back into the fray and opened the fight with a frag grenade.
Long story short, it turns out I was just a little more capable of surviving point-blank explosions made by mis-aimed projectiles than the Legionnaires were. I walked away from that fight bleeding and stumbling, completely out of Stimpacks. But my head was held high.
A proud NCR. You know i was once like you.
Mine was not that epic, but more like random. I had a grenade in my hand, and was having fun near a Viper camp, throwing it inside the camp, and instantly killing everyone there. Funny thing is, my explosion skill was only 30, and i was on very hard.