I just had a moment that represents my fastest sudden immediate bit of FUN ever.
I had a tundra fort, entirely underground, except for a wind farm, and that just powers a bunch of pumps, nobody actually goes up there. It had a population of around 80ish after about 10 or so deaths over the years. It has a booming community. I am setting up a magma chamber up near the sand and clay layers to streamline access to magma, and avoid problems with magma creatures. Along the way, a few other things were to be built, including an obsidian farm attached to a series of levers to control the magma and water flow. I have the entire system going, but just so I can keep track, I haven't broken into the magma yet. It's all set up. But first, I need to break into the water in the underground cavern and close the flood gate behind the miner.
I misjudged water pressure. Within less than a second of the miner breaking in, the entire chamber is flooded to a height of 7. Furthermore, all the lower levels below that (this is the first cavern level) immediately flooded.
It's okay, though, my two miners drown and one glassdwarf who was manufacturing on the lower level. No harm done.
Except that the bottom of the magma pump system was still open because I would eventually need to break ground there to let magma through, and as I said, I had left the whole system on and powered by a bunch of windmills above. The water from above flowed into the magma system at lightning speed, and was immediately conducted from the very bottom layer (the magma layer) alllll the way up to the top of the fort, which is still an unmined layer, and fills in all lower levels quickly and immediately. In literally three to five seconds of breaking ground in the water area, everybody in the fort - literally everyone - is in 7/7 water, and nobody is anywhere near the switch to turn the system off. My dwarves die to the dwarf, as well as all my animals. I've never before seen a fort so quickly destroyed. It was quite impressive, really... and it was definitely a... FACEPALM moment!