I've always been a bit of a fickle player always willing to abandon a fortress for the slightest reason but I've just managed to get my first catastrophic failure.
I found a nice site with just about everything short of a magma vent. It has HIDDEN FUN STUFF though so I can use magma eventually. I started the map to find that most of my map was a single windswept plain of limestone just waiting to have a huge fort built up on top of it. I built a channel around the entire map and put a guarded bridge over it. My dwarves prospered and soon the fort reached a population of 124. The goblins sieged and half of them were killed or captured by the traps at my entrance and the rest just milled about unable to find their way into my fort since my entrance drawbridge was up.
Unfortunately the goblins proved fatal without ever meeting my dwarves in combat. They distracted me long enough that I didn't notice that I was out of usable food. I set my cooks to make roasts out of the 500 or so units of tallow that were sitting around, but it was too late. Half my dwarves were hunting vermin. Since there were something like six pet cats in the fortress the vermin hunting was a death sentence.
Dwarves started dropping like flies and everyone quickly went insane. I dropped the drawbridge to let the invaders finish things off quickly only to have almost the entire population of tantruming dwarves pour out onto the plain along with the dwarven merchant caravan who I had trapped inside when the siege started.
Half the remaining goblins succumbed to the merchant guards and desperate wrestling peasants before the siege lifted, and the surviving goblins fled.
My fortress was a stinking charnel filled with maddened screams of the murdering and weeping. Then the world broke, the game crashed, and when I brought it back up it was like it was all a dream.