I once had a 3 z-lvl tank of water underground, to provide water and power to places that needed it. As such, I had a door at the bottom for access when the whole mess was drained. I didn't forbid the door nor did I lever link it - I didn't think it was necessary, as there were no tasks that a dwarf could have at the bottom of the tank.
Then someone who was on break decided to enter the room. They met with a wall of water, their corpse blocked the door from closing, and the entire fort was flooded (the tank was supplied by either an aquifer or a river, and was some 25x25 tiles large - a total of 75 cubes of water, or 525 units of 1/7 water). I just imagine the moment of time when the door gets opened and the dwarf sees the wall of water.
Bonus points for fish in the water, as I know fisherdwarves were fishing at the time - I got job cancellation spam from the incident.