Let’s forget about the Oops TPK, but let’s discuss about the time it ended-up greatly, no matter whether the party decided t sacrify their life to save the world (or simply their honour) or that horror game where the PC found-out too late that they’re hopeless and bound to die
I have a few not-quite-TPKs stories, but my DM (despite trying very hard to pose as a tough-as-nails juror) is physically unable to kill off his players.
Last time he had the Pally’s god come down and give us 10 HP each to allow us to survive the encounter and run away.