3rd fortress since I started playing DF
It was going really well and after killing a snow flying titan in one hit then a giant, I decided to build walls around my fortress entrance with a water moat and retractable bridges. Once the moat was done, I made a small channel to a nearby river and made 2 pumps to throw the water in the channel to my moat like this ( side view ):
PP77
PP77##777777777777MOAT
77######################
PP being pumps, ## ground, the moat being on the right, the whole thing powered by a waterwheel.
As soon as the top pump started, 2 goblins ambushes popped on the map and I had to set my civilian alert on so that my civilians would go to my danger room, while my small army was working on preventing the goblins from breaching the fortress so that I could get the bridge up once every citizen was safe... Then from there began my first siege.
Now I've got 12 archer goblins around the map moving together preventing my 10 badly trained and equiped military dwarfs from approaching them without being torn to shreds. As my fortress is self sufficient I decided to close the surface and go into hiding.
After a few days, I discovered that once the moat was full, the pump started pumping into the nearby plains, covering a 30x30 square with water and... starting to flood my fortress through a small hole in the west wall, also before I closed my bridges an elven caravan was in my trade depot, and they started wandering in my fortress.
After several months, the elvens merchants went berserk and I had to kill them while their horses are rampaging around the fortress, killing pets and demoralizing my dwarfs while my military is chasing them around. At the same time, the water has flooded the entrance of the fortress were I can enjoy 3/7 of water gushing against the door, the very large flooded surface killing my FPS due to all the water movement, the whole thing is helped by a nice "catsplosion": I've got around 200 tame pets running around the place, killing my CPU with pathfinding and stuff.
And finally my mayor has started a campaign of impossible tasks to do so my captain of the guard "UristPickArmedLegendaryMiner" is decapiting peoples in their sleep.
I started the countdown, i'm pretty sure that my fortress is gonna experience it's
doomfun pretty fast now