My fortress has been running for three years now. There were something like 140 dwarves, some kind of military, running industries, enough food and all that. I had repelled a couple of sieges relatively successfully, killing some cave dragons and goblins.
Then there's a new siege. I order all people inside, and close the gates when the invaders get close enough. I lose something like ten dwarves there, but that's just life (or death, as it happens).
Then the dwarven liaison wants to leave. I'm all for it, because, well, there will be a new one, so I operate my bridge airlock in my entrance corridor and let the liaison out. When the cave dragons outside see him, I realize that my mayor has decided to escort him to the outside.
Quickly I order the bridge levers to be pulled so that the inner lock is open and outer is closed. After some frantic action in the corridor I check and see that yes, the outer bridge is still open, as is the inner bridge. Why? Wasn't the lever pulled?
Yes, it was. Only problem here is that cave dragons are large enough to prevent a raising bridge from working. *facepalm*
I did manage to close off the fortress with only eight dragons loose in the fortress. After they killed most of the active military, I started a recruiting process and they beat the dragons to death. Death count was a bit over fourty after the siege. Now most people are depressed, but only one of them has berserked.
Just after the siege was over a wave of new migrants arrived. I can just imagine that they began to reconsider their decision to move to my fortress when they arrived: outside there were just dead dwarves, the entrance corridor was covered in blood and goblin and dwarf parts, and after they got a couple of levels down the first living people just sit huddled in a corner next to cave dragon corpses, and mutter "Blood. So much blood."
I put them to work immediately. The coffin business is expanding a lot...