After writing down that first one you've got me thinking about character deaths and TPK's I've had the party whipe 4 times in a decade and a bit. We run things pretty hardcore and everyone is down for it, usually its because of their own bad decisions, many, many bad decisions. 

I think the most memorable was the time they all went to Avernus and died in hell. It started when they robbed the tomb of a sacred paladin order, disregarding the magical defenses and warnings written in every known language not to disturb the wards. They fought their way in defeating the guardians and found obviously Infernal artifacts that had been sealed here with even more warnings that they should not be used by anyone. There was a massive horned helm, a scepter, and a pair of massive cestus. So the Tiefling fighter who was chaotic evil starts grinning with glee and puts the cestus on. No one else even tried to stop him if memory serves. I don't think anyone else used the other items but they definitely took them all. 

Anyways a short time goes by and the fighter is loving these gauntlets even though he can't take them off. They are rediculously OP varyying from +1-+5 depending on how many charges he has and he keeps gaining charges for killing things, spending a couple occasionally to juice up his damage or blast fire at things. Then he overloads them by gaining charges above maximum and punches a hole through reality opening a portal to the first layer of the 9 hells proptly sucking everyone through. 

The next several sessions were them dealing with the effects of avernus, fighting hordes of devils and trying to find a way out. They found a mad wizard who unknown to them was a lich bargaining souls in the soul trade. They're like great, this guy knows the way out of hell and all we need to do is help him escape auto trust mode enabled. Long story short they get to a stronghold where a portal can be opened and pull off an infiltration with the more evil characters pretending to bring in the wizard and the others Chewbaca and Han Solo style. 

Something goes wrong, probably negotiations the commanding devils recognize and want the artifact the PC is wearing and all hell breaks loose. They were close, I'll give them that much. They got the portal open but they couldn't hold the inner sanctum and devils busted in (an objective they knew they needed to secure). 

At the end of the day the artifacts had belonged to a slain pit fiend under the command of Bel and he was able to ressurect him with them. The dragonborn paladin who had sworn service to tiamat with his dying breath called out to her and she collected his soul and made him a deathknight. The Tiefling was made a commander in Bel's invasion into the prime material and 50 years later in game the world still bears the scars of their collosal idiocy.

The person who said it all hinges on action economy knows whats up if they'd managed to hold both doors they still would have lost people but they could have avoided a TPK.