Way back when I was making the magma forges in my new fort (Letmosashock, Cobalttimes, a volcano fort), we dug a lava pipe to the forges, which had two drains for emergency burning rock release. The first was off the side of the map many levels down in the usual "smooth and fortify" manner. The second was what I was thinking of as a "joke release" (I remember Boatmurdered) through a long pipe that then went out of the side of the mountain above one of the two fort entrances, to make a fiery waterfall I never intended to use. It had a magmaproof floodgate, with magmaproof mechanisms, and a lever deep in the heart of the mountain connected to it. Because I'm a paranoid player, I also had a vertical grating just outside the floodgate. I then pretty much forgot about it. So did the dwarves of Cobalttimes, far as I know.
A decade later, the latest wave of goblins arrives, and they bring a dozen trolls. I don't really pay much attention, as they attack often and we survive easily, although trolls are new. We follow the usual protocol: the dorfs block off the trade entrance around the side and the military are summoned. The enemy will have to enter through the main entrance, which is full of traps. Go for it goblins, we've got a military well covered in steel on the other side of the doors and traps. Have at thee!
...They didn't go for our entrance though. Perhaps they knew about my steel-clad military. They all trooped up the mountain, and wandered in little circles, while the trolls walked down a long-forgotten pipe above the entrance. I watched them for a bit, realised what was happening, and sent someone to Pull The Lever. By this time, the trolls had taken out the grating, and were heading for the floodgate. The whole thing was under rather a lot of pressure....
I thought at first that the Lever Had Been Pulled, until I saw my magmaproof mechanism surfing the front of the wave of smoke and fire and charred furry loincloths. Oh dear. None of the enemy survived, as the few horrified survivors who had been further from the pipe sought shelter from fiery death by cowering in my entrance, and dying in my traps. The military was not needed. It was all so fast that I didn't even consider taking some screenshots.
I now have a permanent red burning river. I cannot turn off the flow. The lever, of course, does nothing. We are digging a drain to let it flow off the map, and a bridge over it, and a new trapped entrance.... Without a magma trap. But if I did instruct the dorfs put a new one in, it would have a lot more gratings and extra floodgates, and vertical bars, between it and the great outdoors! Or I would arrange, somehow, for the mechanism to be placed on the INside of the floodgate, because that was evidently the mistake the mechanic made!