I found a lovely natural waterfall when I embarked, with the water falling deep underground (about ten levels) and post-waterfall brook going out the map underground. Very beautiful indeed... I immediately started planning to use the brook for power and water.
With my fortress just turning 2 years, I was on the last stages of harnessing the power from the brook with waterwheel farm (water and irrigation was OK already). Everything was ready and all I had to do was finish it, letting the water out just next to the waterfall (back into the brook) and pull the levers to let the water flow.
The chief (expedition leader) and the cook, both legendary miners, and also lovers, volunteered for the job. It was supposed to be simple channeling job, no biggie. But at the last minute, I changed my plans as I realized I didn't have to go above the ground to let the water out of my tunnels, but I could dig straight into the waterfall instead and still be safe, or so I thought... Miners went at it, and then all
hell mist broke loose. Water flowing, a whole lot of mist everywhere and I couldn't see my mining-duo anywhere...
First I didn't know what happened. Then I realized that the chief was dead. It took me a moment to pause the game, and then I found the body under the waterfall, ten or so levels down...
But the cook lived to tell the tale. After witnessing terrible death of her lover, she was comforted by a lovely waterfall, shrugged it all off and fell asleep
true Iron Chef she was - Smashed leg and broken finger, but nothing too serious (yet), and after she woke up, she made a stairway up for herself, and once on the surface, she limped towards the hospital...
...but the above was not the face palm moment
it was when I saw a message that my "doc" had died "after falling and meeting a solid object"
I forgot to cancel the mining plans that caused the whole waterfall mining accident and the good'ol Doc wasn't too scared of the chief dying and seeing the cook limp out of the mist with bloody leg and hand, but rushed in to finish the job...