I decided to dam a river so I could have the entrance to my fortress floodable when elves nobles goblins came. To do so I decided to drain the river so I could build the floodgates. I built a room with a staircase in it that led down two z-levels to another drainage tunnel to drain the water into a cavern. I forgot that when you dig a staircase and pour water down it the water ends up on the floor below the last floor on which you build the staircase. I was halfway through construction of the room when I decided that this was about time to tunnel out the dirt next to the room's floodgates, exposing the floodgates to water. However I forgot that water flows diagonally too.
This is kinda what the room looked like:
XXXXX~~~~~~~~~~
XS F ~~~~~~~~~
XS F~~~~~~~~~
F~~~~~~~~~
X X~~~~~~~~~~~~
~Water
X Stone/Dirt
F Floodgate
S Stairway.
So then the water flowed into the room, out into the other rooms. So I thought, well at least this is all I have to clean up. But no, the water flowing down the staircase flowed into the room below the staircase, flooding at least 4 large rooms. I tried to stop the flood with doors and walls, but to no avail. So then I decided to build another room like that one, which actually worked, taking all the water off the map, despite halving my framerate. But the river had been drained, so that was good. After a series of failures, however, I managed to flood it again.