A few of my luckiest moments include my last fort, prior to succumbing to FPS death from getting sieged by almost 100 goblins/trolls+mounts, was the sheer success of my checkerboard traps. basically... I build on long, winding 3x3 corridor leading to my fort and place traps in a checkerboard fashion because while being very inefficient... it's positively hilarious. For the first 2-3 sieges almost all the combat logs consisted of little more then dozens upon dozens of Goblins getting torn to shreds by iron discs, silver balls, random weapons I didn't feel like melting, and stone traps I put in just because. Favorite one was a 50 goblin siege where the lead Goblin ran first, got eviserated by the first disc trap, and the siege broke and everyone else fled. It was lucky because most of my military was recovering from a Forgotten Beast fight that left most of them wounded due to symptoms.
Another lucky moment that was quite amusing was in said previous fort I was digging down to find the caverns, my miner began to dig a downward staircase... and fell 10 Z levels into the cavern, landed in water, broke a leg, fumbled out of the water, gored a Giant Olm and 6 Crundles in the face with his pickaxe, and casually limped to a spot I told him to dig to start his march back up to my fort, and proceeded to gore every hostile wildlife that came close that dared to interrupt his trek back to comfort. He then got eaten by a Giant Cave Spider.