My favorite (which I think I've told a million times) is the great blind cave ogre rebellion of Wrungknives.
Wrungknives was my fort two versions ago, it was an attempt at an above ground challenge (I ended up setting up a hunting ground and noble quarter in the cavern) a city and a gateless challenge. It succeeded enormously and grew to cover the entire surface of the embark with a red clay sprawl of a city, complete with three pyramids over the holes where I tried (and twice failed) to pierce the aqueduct.
My hunting ground was also a success, I played around with the raws and made all the cavern creatures tameable. Somehow or other, the only successful breeding program ended up being the blind cave ogres. I bred them until my animal population hit the caps. My duke had two personal blind cave ogre bodyguards, as did almost every crucial dwarf in the sprawling, 50 year old city of Wrungknives.
One day, I decided to try the dfhack function to switch from dfmode to adventure mode and see my city from an adventurer's perspective, without it being abandoned and empty of friendly life.
The moment I did so, switching to my finest warrior, every last cave ogre in the fortress went hostile.
This was probably my favorite adventure mode session ever. I ran wildly through my sprawling city, periodically running into a friendly dwarf under attack my a cave ogre and having to help them. I told my self I was supposed to rescue the duke, but I spotted the duke exactly twice, once when he help me by one-shotting a cave ogre with his crossbow, and once when he ran past me on the floor above, probably towards combat.
I ended up dying to an unlucky dodge, but it was a really fun and highly unexpected and unscripted turn of events before that.