Fancy system running off of an underground river designed to both be a perpetual motion machine and an addition to the dining room waterfall. Overconfident with my experience I don't build a failsafe lever into the system. There is no way it could possibly go wrong.
Unfortunatly the dining room waterfall already had a waterfall, sourced from a surface river (also without a failsafe). With an expertly desined drainage system that emptied into the underground river. It was working fine.
Until I added the extra water without expanding the drainage, that was. Now there was twice as much water going through the system, the drainage system slowly backed up, not able to pass all the water through the thin drainage pipes. Then after a quick glance to make sure the dining room didn't immediately flood I pat myself on the back and forget to pause before going to make some hot pockets.
When I get back delicious hot pockets in hand, the dining room is under water, water is flowing out of the fortifications into the 2 z level tall workshop hallway, and the poor souls in the jail have a wall of water flowing down the stairs at them.
In the end the jail level is completely flooded, as well as a few z levels above it, that had nothing of note, the dining room and most of the workshops are knee deep in water that wasn't going anywhere soon, and after the pumping system was hastily deconstructed it would take years for the drainage system to catch up to the extra water in the system(because I certainly can't shut off a direct waterfall from the surface river). Not in the mood to salvage the disaster, I abandoned.
Deaths: 3 residents. 2 that drown in the jail(ignoring a mandate from the baron and mayor) and 1 that apperantly got washed into the well and drown. theres a facepalm moment right there.