Gather round children and let me tell you an epic tale of glory, wonder and.....glory.
In my latest fortress, Iridastesh, more commonly called Rhythmiccudgels, I decided to mine out an exposed bauxite vein visible from the beneath the ocean, but a few z levels above most of my fortress. All was going well until I accidentally mined out the wrong piece of rock without realising. A tiny gap to the ocean was opened without my noticing and seawater rushed in. Finally I started getting messages of cancelled actions due to dangerous terrain and I sprung into action designating a wall to be built to stop the incoming tide and ordering all the accessible bauxite to be brought to safety.
Problem solved I thought, but no after building two of the three pieces of wall to block the corridor, all of my masons (about ten including all the amateur peasant masons in my block making gulag), decided to take a simultaneous break as the water flowed steadily closer and closer to my defenseless fortress. After a few tense moments the inevitable happened, the water reached my unfinished wall, making it unbuildable due to dangerous terrain, my fortress was lost I thought and was ready to give up.
I then realised that my wall however was slowing down the water enough so only a trickle was getting past, and after a few modifications involving an alternate path for the water to flow down and a floodgate for good measure, instead of a watery tomb for my dwarves, through blind luck I had instead created my first mist generator right down my main staircase, making all my dwarves ecstatic, despite the fact that in reality their fort was extremely slowly filling up with seawater.
Now instead of a normal fortress with no real special features which I was on the verge of quitting I have a fortress slowly filling with water (I estimate years until it even fills my bottom level) and a mist generator down my main staircase. Does anyone else have any stories about horrible mistakes that came so close to disaster, but instead had awesome unexpected consequences?