I presided over a TPK once, and it was player-driven - sort of.
The group found a "backdoor" into the dungeon, avoiding walking right into a red dragon's ambush. This alternate means of entry was a near-vertical lava tube. I had them roll to notice that the sharp edges of the lava could potentially cut their rope. One player (unbeknownst to me) interpreted this as a "sign from the DM" that they should not go that way. He convinced the others.
So they went in the front door - and died to the last.
The group found a "backdoor" into the dungeon, avoiding walking right into a red dragon's ambush. This alternate means of entry was a near-vertical lava tube. I had them roll to notice that the sharp edges of the lava could potentially cut their rope. One player (unbeknownst to me) interpreted this as a "sign from the DM" that they should not go that way. He convinced the others.
So they went in the front door - and died to the last.
