Well, once I ended up pumping water out of a square faster than it could flow in, in order to install a floodgate.
I'd dug out a square next to the existing gates, in order to improve the flow rate for the prototype elf drowner, but forgot the important part of drowning elves. The water flow. So, my trade depot flooded. The entrance to it was full of flowing water, going through a gap I couldn't plug without the water not being there. So I pumped it out, with the addition of a miner wading into the flow to channel into a lower tunnel I'd dug. Which wasn't really intended, as I'd just meant to give the water somewhere to GO, not almost flood the bloody fort.
My noble-killing chamber in one fort ended up as a platform with bait levers on, supported by a support on a thin strip of cantilevered land below it, with access via a glass bridge.
When the lever elsewhere was pulled, the support went off, crashing the platform down through the bridge and into magma. It even sucked an elf diplomat into it at one point.
The first WORKING elf drowner worked through active pumping from a tunnel dug from a brook. However, the water drained back into the brook again through the grates it was being pumped through, so active drowning was only acheived through constant pumping and pressurisation. Even worse was the way it all flowed back into the tunnel after use, into an already-full brook.
Then physics got themselves worked up, and the indoor wells flooded because they were on the same Z-level as all the brook water and hooked up to it by channels. But only after the drowner was used.
Physics also cries when you knock bleeding creatures into flowing water. The blood goes right the way downstream, even soaking into the banks.
I once diverted a brook down a few Z-levels, into a dead end. Physics did NOT cry, as my waterfall overtopped and flooded the landscape, as did the fountain from an un-doored escape tunnel. Physics kicked my ass, but I got my own back when I used it to extinguish a burning dwarf.
Who then died of his wounds.
Titan in a wooden cage.
Fire imp in a wooden cage.
Quantum dump piles of unwearable clothing. Hundreds upon hundreds of articles. In future, I should just use a shaft to stick it in the middle of a gigantic stockpile and worry about bins later.
Oh yeah, and cramming 100+ large animals into a single wooden cage. Although, if it's the same kind that traps titans, it's not surprising.