I lost epically today-- I was following the Lovecraft thread and tried to build an large, magma spewing Cthulhu (octopus-dragon) into a volcano, whereupon my dwarves would dwell in comfort. Since my last fort was a humdrum 17 year fort guarded by 30 blue-equipped legends I had utterly forgotten that goblins do more than explode into bits upon encountering a member of my military. I also discovered that sieges can come after your population has already dropped a whole bunch.
First, to stop a raid, I blew open a magma chamber near my farms sealing half of my fortress away with magma. Unfortunately the goblins were in the half with all the food. And as I mentioned the farms were knee deep in magma. So I sent my entire population to beat the goblins to death. We made it, losing something like 30 dwarves in the process. Now I have 50. Then the tantrum starts. I now have 25. A good portion of those are mad. Then another siege comes! Many, many crocodile riders. So I pull the levers--magma pours out of all of Cthulhus tentacles--three dwarves melt as do a bunch of goblins. Magma is running off the mountains and into the real world. Somehow the bastards make a beeline through it and kill everyone but a child, who runs around a while NOT pulling a lever which would seal off the volcano (children!). When I tried to reclaim--still a dozen goblins and every square inch outside my fortress is covered in magma. After one f.p.s. death after another I consider this a win!