Years ago, I was running a D&D 5e game, and I decided I wanted an adventure with pirates. So as I often do when running a D&D game online, I started by googling for a battlemap that fit my theme, and I found a three layer ship map. The way the ship was set up, I could easily set this up as basically a linear dungeon that would lead to one small room on lowest deck of the ship. So I set up a bunch of encounters on the way, including a joke one in the kitchen that was right before that last room (I just put a grung who would be annoyed that he was being interrupted before ignoring the party and resuming cooking), and in the final room was the boss, a big frog monster, his lieutenant, and a tied up mermaid the party had to rescue. Basically, it looked to be a perfectly fine but probably middling D&D adventure, at least before the party smacked it in the face.

So it starts off pretty normally. Most of the party goes to the top deck saying they want to join up with the pirates, and the monk decides to see if she can sneak ahead a bit to scout. That made sense, I thought, so I had her make a stealth check, she easily passed and got a look at what was just below deck. Then she said she wanted to go a bit further. Another good stealth roll. A bit further. Another good stealth roll. She passed every damn stealth check until she got to the joke encounter in the kitchen. The one that's right next to the boss room. And it didn't make much sense for that grung to act differently, so her one failure so far didn't fucking matter. And of course she opens that final door, trying to make a hard stealth check, and she botches it.

Me: You open the door, and the three occupants, the large frog monster you know to be the captain, his lieutenant, and a tied up mermaid all stop what they're doing and turn to look at you. What do you do?
Monk: "Oh, hey, I just got a job at the kitchen, and I was just double checking that you're ready for your food."
Me: Make a bluff check.
Monk: rolls NATURAL 20 BABY!
Me: ....the captain says "How many times do I have to tell you people to tell me when you hire new staff, even if it's just for the kitchen!?"

And so she decided to help the grung out in the kitchen, and I made her make some roll for that, and she passed. And then she reminded me of a decision I'd made a session or two before that. You see, they fought a few giff, and the giff have an attack where they throw a grenade. Technically it isn't presented as an item the players can get, but I had allowed them to take a couple from the giff who they defeated who didn't use that attack.

Monk: OK, now I pull out those grenades we took off those giff and bake them into the food.

I paused, but decided fuck it, give it a god damn shot. I don't remember what I had her roll to get them in there, but she of course made it, and then she passed the bluff check to convince the captain that nothing was up.

Me: So the grenades go off in the captain's belly. I'm going to say the effect of that is that he doesn't get a saving throw to reduce the damage, and the surprise of that means he won't act in the first round of combat. We're in combat now, and you're up first. What do you do?
Monk: Wait, she's a mermaid, right? She can breath water. I'll just punch through the side of the ship, and we'll just swim away together.
Me: ...I...that...
Monk: I'm a martial artist. Breaking wood by punching it is what I do.
Me: I...guess I can't argue with that. Give me a roll.

And so the monk punched a hole in the hull and swam with the mermaid as the ship started sinking, and the rest of the party above deck along with the prates realized the ship was sinking and made their exit. There still was a boss fight with that frog captain, as he could swim to shore, but they made short work of him.

So what did I think when that was all over? That is was the best D&D session I ever ran. What I got was a bizarre, awesome story where legitimately no one, not even me, knew where the fuck it was going while it was happening. It's the first thing I go to if I'm with people who are telling D&D stories. If things had gone to plan, my story would have just been "oh yeah, I used a pirate ship like a dungeon once," and I'd probably barely remember any details at this point.

In the moment, it can be be a little scary when things start going off the rails. And there is of course part of you will feel like if you set all that up, you need to use it. At times, I was tempted to figure out something that stopped that monk's progress artificially or caused a fight to start above that would bring her back or something. I could easily have said "No, we're doing this the regular D&D fight way" or whatever at any of the wackier stuff. But by letting the players just poke at the seams and trusting myself that I could work with whatever happened because of that, I got something special. I feel like this session is when I started becoming a good DM.