This just happened and I feel the need to brag.
My fortress was at well over 100, with only three deaths from sparring, no criminals, and (almost) tolerable nobles (No export of tin? Never even seen any, sorry.) Despite being in an evil region all snug to a goblin fortress, the only real danger seen so far was three different bands of dark gnomes.
In my fourth year, a dragon attacked. I immediately ordered everyone inside, and once they were in closed the front drawbridge and readied the ballistae. The lever was pulled, and it was good.
The bridge never closes. Nobody ever shows up to man the ballistae. The funny part? The dragon strolls right by my undefended main entrance, strolls up to a side door that's used to access a mined-out magnetite vein, mauls someone's pet mule, and loiters around the locked door. I order my military squads next to the door, preparing for a dwarf rush, when I notice the lone marksdwarf who decided to stand on the side of the wall WITH the dragon on it. With NO AMMO. Seven of my eight champions walk off at this point, giving various excuses about getting a bite to eat or a quick nap. My last champion, an elite wrestler, proceeds to attack the dragon (who's almost finished ripping that marksdwarf limb from limb) from behind with his bare hands. He kills the dragon with his bare hands, unassisted, without taking a single scratch.
In about a minute various Dragon Tallow Soaps are being hauled from the scene. Seconds later, a weaponsmith becomes secretive, and quickly begins a mysterious construction.
Less than five seconds after this, my miners strike adamantine, in a site I wasn't even aware had it.
On their own, none of these events are particularly notable, but they all happened in less than two minutes. Seriously. Talk about unlikely.