Well ... I have had a number of near tpk or tpk incidents that I simply never would have expected. Most memorable deaths aren't always in the TPK realm though.
Right after 3e was introduced the small group I was DMing for ran into some orcs. One PC had found some poison (very deadly, highly illegal, and big hints were made to destroy it, but he was neutral, so ...) so he decided to poison some arrows, to make a long story short, he shoots into melee (without the feats) "hits cover" -the cleric, who fails his save and ends up negative the next round, and then poisons himself with a 2% on the roll to apply poison. I think the poison did 2d4 Con, he rolled max and only had a Con of 8 to start with. Basically, the rogue took out himself and the cleric in a simple battle that, quite frankly, they could simply have avoided (the orcs didn't see them).
We all roll on the table in clear view (including me when I DM) so everyone knows when I have a crit. Normally, as DM, I roll fairly poorly, but I get into "hot streaks" that can make players pretty nervous. Combine that with the PCs missing, well, strange things can happen. When a PC decided to climb a tree and "mess with" an eagle while the rest of the party stood back. The eagle proceeded to get a total of 5 confirmed crits plus several other hits while the PC could not roll higher than a 4. Those d4s add up pretty fast, the eagle brings him down to -1 hp and he falls out of the tree. I let the player roll his own falling damage with the kind decision that even though he was 60 ft up in the tree he would only take 2d6 damage due to branches breaking the fall of his unconscious body. He rolled 6's, he has never lived down that character death. We live in different states now, but his group is still known to say "Look, an Eagle's nest!" Another character death brought about completely by the character, I mean, he could have started retreating at any time but refused.
Patrick