We had several memorable ones in my games. Sadly, most of them game from one player.
The most recent was my own, and it amused me to no end. We're playing an EQ game, and at 1st level, ran into a diseased rat. Apparently there's a disease template, adds 1 cr, highly dangerous. Made this lil' bugger amazingly strong. Between the 5 of us, we managed to slay the rat, but one character got infected with a disease that halves your Dex, lowers your Cha to 1, but gives you +4 str, +4 con, and dr 1/4 your HD. Not bad.

Course he was highly infectious, and caused people to want to kill him on sigt. We tried to get him healed, but eventually, being an evil party, I decided that 1. He had done perhaps 10 points of damage over the course of 3 levels, so wasn't much use and 2. It would cost more to heal him than he was worth. So I attacked him, when he had 6 hp left. My troll had 38 hp. I rolled a 2. He critted with his greatsword. Down to about 23 hp (high str). I rolled a 4. He critted. Max damage. Down and bleeding, he harm touches me, and I quietly expire.
First thing he'd killed all campaign, and it was me....
The rest of them all came from one player (a friend of a friend of a friend) who just wasn't too bright. His first death came when the party went on a scouting mission to investigate a band of orcs that was rushing the city at a high rate of speed, more than seemed possible. The party mounted the fastest scouting horses we could find. Except for his paladin, who mounted his heavy warhorse, in full barding. Predictably, we found the orcs, and they're fast (50' movement). Our horses run at 50'. His runs at 30'.
You can guess where it went from there.
He later died charging a palisade which contained approximately 200 orcs, armed with bows, who had setup spiked embankments. He actually managed to reach the top of the wall, by spending 2 hero points, where he found himself surrounded (surprisingly). I think he was probably tasty, though.
We were glad to see him leave the table....