I felt very close to my first-ever TPK last night.
The party have been steamrolling almost everything I've thrown at them to date, so I deliberately built a slightly vicious encounter to test them (eight CR 3 and one CR6 against a party of 5 level 4s). The basic setup was that an orc warchief they have a bit of history with was undergoing a ritual to ascend into the service of one of the orcish gods; but the thing was set up so that they had the opportunity to stop the ritual in progress, and make things easier for themselves.
Instead, they split the party.
The druid, bard, and rogue decided to take a short rest a quarter-mile from the ritual site (they'd done their usual Full Nuke on the previous encounter) while the fighter/barbarian and sorcerer/paladin snuck ahead with 30 hp between them (along with an orcish defector as guide) to scope things out. Inevitably, they got spotted by one of the orcs, who took out the sorcadin in one hit.
Fortunately, the fighter/barbarian had a sending stone, so he was able to call for backup…
…in 12 rounds.
And somehow, they almost managed to hold out. The sorcadin rolled a nat 20 on his third death save, bringing him back into the fight long enough to use both a scroll and necklace of fireballs. There were a bunch of low attack rolls from enemies, and a very lucky/clever use of a deck of illusions-summoned "cloud giant" to scare off two-thirds of the remaining orcs.
All three of the characters in the ritual circle - NPC orc, sorcadin, and fighter/barb - were at zero hp when the druid and bard arrived (the rogue had stayed behind to finish his short rest*). Everyone had managed to make enough saves to stabilize, but then the druid got knocked out, and the Big Bad - by this point the only remaining enemy - kept saving against the bard's spells. He would have KO'd her, too, but she pulled out a Cutting Words against one of her attacks, then healed the druid, who managed to deal the final blow.
I was hoping for dramatic, but that came closer to the wire than I expected. I'm glad they all made it through, and there were some big cheers when the luckier rolls came up. Looking forward to a more chilled-out session next time, though.
*This was in part because the player had a 5am start, so left the session early