So, my only case of a lost fortress occurred while I was online chattin' with Sava. I had these orcs show up, to my first anti-orc fort. I had all kinds of airlocks and shiz to shut off parts if it were invaded. I set up a magma flood for my entrance, and two bridges for stuff to get caught between.
Of course, when testing, I couldn't get any dwarf to pull the levers. It was an infuriatingly bad case of "choose the worst possible dwarf, and make sure he decides to go get a drink when he almost gets back to the meeting hall and lever". As a result of this grabass tomfoolery, an orc macelord got through.
A glowing blinking blue god of death. To top it off, Dr. Manhattan's name was about 2 pages long, and all of my dwarves were scared shitless of him because he'd been killing members of the civ in world generation. Anyway, I had decided to let them reap their reward for idiocy and insubordination, so I didn't savescum. I recruited the 34-odd adults and stationed them where Dr. Manhattan would shortly be coming down the corridor. He was still maiming random dogs, flinging them into walls and shit, by about the time 20 of them get assembled. He finished that and came down the hall for my dwarves. Lo and behold, only a few of them work up the courage to fight him at a time! With the various legendary craftsmen/recordkeepers I have, it's a cinch that they could take him down in a mob along with my relatively weak military.
But no. They have to go at him a couple at a time, while the rest generally flee in terror, then work up the courage to attack him when the others have died. Eventually, it becomes a case of a fortress full of terrified dwarves fleeing for their lives, watching their friends dying gruesomely one at a time as they either work up the courage to finally fight or get cornered and turned on their attacker.
It was the most pathetic display of dwarfsmanship I have EVER witnessed.