Our bard got a magic item that let them cast Wall of Thorns before we would normal have access to a spell that high level. We used it on a boss pretty much immediately to trap it in one side of a room while we mopped up the rest of its adds that didn't get killed by the Wall. The boss we were fighting was a bug queen. What we didn't know was that, while we were fighting the first adds, it spent the whole time spawning more behind the wall, since it didn't have any other actions it could spend. That did not go well lol

It was our first TPK, most of us were new to the game, and we put a lot of focus on RP, so the DM gave us the choice if we actually wanted each of our characters to die, or if we'd prefer to be Deus ex Machina'd. I was the only one that chose to let their character die. My warlock had made his pact initially as part of a deal to save his sister who was crushed in a cave-in. He decided to swear more oaths, fully giving his soul for eternity to his patron, in exchange for saving the lives of his friends.

It was a pretty cool way for him to go out, and now he gets to be an antagonist later. That's going to be interesting, because our bard took up his fake religion out of a desperation to know that he went to a better place

I was the cause of a TPK as a party member. I am notorious for my bad rolls, to the point it made my character useless and myself as a player unable to really involve myself in any rolling interactions. So to combat this we homebrewed a character for me that was a sorcerer who was obsessed with the arcane and made a warlock pact with a chaos demon.

This pact gave me great power at a cost (Meta >3 was double damage 4-20 1/2 damage, and I roll for my spell selection each day, we had a chart, DM approved. I also rolled each cast to see which of a type of spell I'd cast). While the cost was great I also became insanely powerful, unexpectedly so, by everyone. Eventually the party and I were told about a house that people kept disappearing into and how it used to be an old brewery in the basement before the owners just disappeared too.

The party set off! We make it to the brewery a f as soon as myself and another party member set foot through the door some goopy slime falls onto our shoulder. Confused we look up just in time for a massive gelatinous cube, fed by many town people, to envelop us. The monk just behind us goes to start wailing on it as his two party members are helplessly engulfed but stumbles on the entrance and is equally absorbed.

I, a great wizard with a penchant for the insane arcane, decide I shall boil the gelatinous cube. I hope to roll a lesser spell but end up with the only high(er) level spell in my damage selection. Fireball. Roll my homebrew damage mod hoping for 1/2 damage, 2. Double damage fireball. Point blank. Inside a confined (gelatinous) cube. There were no survivors, but great stories never die.

My last campaign we were meant to be fake tpk'd in a fight and wake up in prison.

Annoyed the hell out of the dm when we won the fight as it cut a whole storyline off.

A certain wotc module has a level 2 party fight a suicidal enemy with Fireball in a small room. Also the fireball does necrotic damage just to make sure no one can resist it. Went exactly as you'd expect. I haven't read the module myself so not sure if the DM messed something up or what. I thought it was hilarious but the other players all rage quit so that campaign ended there.

We almost TPK'd on our very first encounter fighting 3 goblins. A party of 4 level 1 adventurers. We didn't even get ambushed. As one member lay dead and another dying, (and the other two of us near 0) we ended the session.
The dead cleric saw their diety, who said their work was not yet finished, saved the sorcerer's life, and expected additional converts to the religion.

Oh a couple. Were much more common in previous editions.

First one I played, a swarm of bats comes out a cupboard I iust opened in a haunted house module. I'm playing the wizard who could burning hands and save us all in an instant. But we'd been plagued by a few illusions, and so I attempt disbelieve it instead. Two rounds later: we're all dead and the party blames me.

First one I ran: smaller party of three in Dark Sun. Just a few halflings to try and nick their stuff, this one should be easy right? Halfings use poison on Athas. Paralysed one, critted the other, last one frantically trying to unparalyze the other party member, and so I basically have to as a DM or this isn't Dark Sun; I swarm him and tie him up. They then proceed to roast him alive, and eat him. Lesson learned: don't mess with anything on Athas, lost more than a few parties on that planet

Only once. DM had a circular dungeon. We started a fight and got split and a couple PC's fled the fight deeper into the dungeon.

Some out of game drama leaked into the game and one player sprinted their character through the whole dungeon, brining all the enemies together behind him as he looped back around.

It was a shits and giggles evil campaign, we weren't, super attached to our characters, so we kicked that player out and started a new campaign.

Tarrasque Point Kills?

So many.

Curse of Strahd in 3.5: Who knew Con damage to a mostly spell caster group could be dangerous not just once but 4 times. In comparison both fights were Strahd were a f**ing walk in the park.

More of a test game than an actual campaign: We were testing if a CR appropriate group of goblins with levels vs a group of players would play out for the players: Well as it turns out I suck at calculating CR so our first fight was 5 players vs 5 goblins level 15 (Same as players) plus 10 starting goblins. We adjusted it for a correct CR and it was still a massacre. Ambush is a HELL of an advantage.

Anything goes campaign. Well turns out anything goes for the DM too so Level 3 half colossal half minotaur barbarian opponent with a posse of gobs with a ballista safely away means players die every other round or so.

One member of the group had decided to leave, so wanted a valiant sacrifice for his character, none of the rest of us would leave him behind though, so we almost got TPKd before the DM admitted there was literally no way to win the encounter.