I made a tunnel from the river on the edge of the map, half way across the map into my fort, to get some mist in my dining room. For security reasons I put a floodgate at the entrance, but soon found out that the levers I built in my fort to control it was too far away to connect. So I build a control room near the floodgate, put a lever there and gave it two orders: Link to the floodgate and Pull the Lever. But nothing happened, no dwarves was nearby.
Suddenly a forgotten beast turned up in the caves, smashed the security walls and began killing some children, then slowly working itself up the central stairway and killing everything I sent after it. I forgot all about my waterfall project.
Just when I thought that I had lost the fort, some dwarf actually fixed the lever and pulled it. And due to some small miscalculation on my part I didn't get a waterfall and a mist, instead I managed to flood my whole fort. That killed the forgotten beast and my military (if they weren't dead already), but most of the civilian dorfs survived, so after a looong time I managed to get someone to close the floodgate again. During the clean-up I discovered that in addition to the forgotten beast I had also managed to kill/drown two bat soldiers, who happened to be stationed in the caves just where all the water left my fort.
That was completely awesome! Unfortunately the fort died a few months later due to a tantrum spiral caused by my new and very green recruits all getting killed during the next siege.