Built a very large fortress. Had produced well over 5 million wealth without adamantine, had withstood scores of ambushes, and dozens of seiges, and four megabeast attacks. Was going great, I was feeling invincible. My main floor level was a huge sprawling stockpile area with furnature, crafts, cloth, leather, etc. Pretty much everything under the sun. The underground river blocked expansion southwards, but I did what I could (Digging in would have flooded everything. It was an interesting feature, starting high and having four waterfalls along the way, one prior to the stockpiles, before falling down a chasm near the HFS)
Downstairs I had the food stockpiles, farms, magma smelters, upstairs the craft shops. Three floors down I had the dining room, then below that there were all my bedrooms (For about 200 dwarves), and the tombs under that.
I decided I wanted to build a self sustaining waterfall for my dining room. No biggie, done it before, so I dug a path to fill a reservoir, floodgated it all up, linked it so I could fill by stages instead of drowning everyone. I dug a channel from the side of the river to let the water in. Water goes down the hole. Water fills the reservoir. Water backs up the huge hole. Water escapes through the hole I didn't notice on the stockpile level. Water floods my stockpiles area with incredible speed. Water floods everything before I can get water stop measures in place. Water floods over the only stairwell. Water drowns a huge proportion of my population who were trapped in the bedrooms with no way out (I have to say that some of my dwarves did admire the new waterfall in the dining room though). Water continues to flow into my stockpiles with great force. Water washes all my stockpiles out the front doors. Water floods the walled woodcutting area. Water washes everything out the gatehouse.
That fort was absolutely unrecoverable. It was awesome.