Well, my 20 z-level pump stack that I built without walls in order to have a huge fountain, predictably, flooded the banks of my river. Since I'm on an aquifer (and they make SCARY amounts of water), the river stayed full and the pump stack kept pumping... Then the water it pumped overflowed what I anticipated and the currents starting carrying random dwarves who were outside for a variety of tasks to their deaths. *facepalm*
Fortunately, I anticipated this possibility and left a lever that could shut the system down. It was blocked by 6/7 water. *facepalm*
Finally, someone pathed right and got there and pulled the lever. It dropped 60 tiles of 7/7 straight into the river, which had a pipe going straight into my fortress so my dwarves wouldn't have to go to the river and die of carp attack. Well, it flooded and my fortress began to get wet. *facepalm*
Fortunately, I had a reservoir for exactly this situation! I pulled the appropriate levers and pathed the water into my reservoir. Where, forgetfully, I had stored all my finished goods which were subsequently washed all over the place. *facepalm*
So I tried to get the dorfs to release the water in the reservoir, but they never got around to the lever-work that would make that happen... *facepalm*
So I deconstructed a wall piece. This went badly, as it flowed down through a Open Space I didn't know was there, straight into my fortress. *facepalm*
And that's my FP story.