Okay this is gonna sound like the most schizo shit imaginable and a little long but.
One day our cleric couldn't make it to a session, and my other players requested I write a side quest for them to do since they still wanted to play despite being unable to progress the main story without the cleric.
One player in particular was VERY adamant about facing the mascot of Celina 52 Truck Stop, "Piss Jug Man", pictured here

So I was like "You know what? Who am I to deny my dear players their desires" And decided to write an encounter with him inside a small diner called "Waffle Manor".
It was indeed a waffle house, as the players stepped in a barbarian and a warlock were having a deadly battle inside but they calmed down after the diner's owner dispelled their magic items.
Gonna skip over some story bits straight to the battle but Piss Jug Man emerged from the totally not Waffle House's bathroom and combat started.
Now here's a little fun fact: I am an idiot. And designed this encounter with "oh yeah our cleric can heal them so it wont be that bad" in mind. Completely forgetting the fact that this whole encounter was made because our cleric won't be there!
The combat mechanics were simple. Every time it's Piss Jug Man's turn, he would spawn slime minions, respawn previously killed slime minions and copy one spell the players used within that turn order. With the cleric this could've been fine since the slimes are all really brittle and the party just had to sustain. But we don't have the cleric so we cant sustain!
The players were immensely intimidated by the Piss Jug Man and thought "okay we can't possibly kill him, maybe if we unscrew his cap we'll win"
I did not plan this at all?? But was like "haha yeah, good eye! If one of you grabs him then the other can unscrew the cap. Do it 3 times and you win"
This is hard since it means our barbarian has to hold him, our artificer has to unscrew the cap and our ranger is alone fending off all the slimes.
They unscrewed twice but finally our barbarian went down. The artificer was at like 5 hp and our ranger was completely surrounded as Piss Jug Man got his turn back and respawned all his slimes.
All hope was lost, then the ranger said "can I aim for the cap to unscrew it?
I told him to go for it. But he had -2 to the roll because he was aiming for something specific and he had to roll above a certain damage to get it.
By some divine miracle, he rolled well both on his attack and damage. The cap came off and the battle came to an end as Piss Jug Man collapsed into an empty costume on the floor.
This was the hardest encounter my players faced until near the end of the campaign, and first real TPK threat we ever had.
TLDR: Nearly TPK'd my party with Piss Jug Man.