- Setting up an aqueduct to transfer water to my fort.
All well and good, but it involved a waterfall. And I forgot to door off some of the escape tunnels.
The flow overtopped the waterfall's bounding walls, and gushed out the escape holes and flooded the area to 2-3/7 water.
That was... Inconvinient. And embarassing.
But then a dragon attacked. A fair chunk of my military was set alight, and killed because they were untrained. I hadn't figured out that 'f' cancelled the designation, rather than assigning it to nobody in particular, so no barracks.
Then goblins ambushed, wiping out the military and a few popular dwarves. The harvests rotted in the fields, the doors were taken by the enemy, and I gave up in disgust. Later, I loaded up and abandoned.
- Trying to drown the elven caravan.
I was using a positive-pressure system rather than pumps. Through my only access corridor.
It didn't work fast enough, so I enlarged it. While it was in use. I dug a hole, the whole thing kept flooding, and the drainage room was filling up, so I shut the door and let the lot flood.
I ended up having to set up an emergency trade depot that was in use for two years while I set up a huge wind-powered pump system to drain the drowning pathway enough to install more floodgates.
In the meantime, the dwarf caravan got stuck on a wall built as the wagon went up a ramp, and went mad. Once I figured the problem out, they got out.
Then there was a huge blighted patch on the ground where I forcibly flooded it with the pump outlets.
Finally, I tried to get more pressure by directly hacking into the brook, instead of using an underground canal and pumping it up. And I routed every murky pool on the damn map into the artificial river. It filled up alright, but EVERY BIT OF GODDAMN WATER ON THE MAP was moving. My FPS died to unplayability.