Be me:
Make grand two dwarf military to be uber elite soldiers.
Armed fully in candy, badass and legendary.
Goblins! Send out the elites! Dine on their blood!
Aand they're slaughtered before they kill anything.
*facepalm*
Why? Cave adaptation apparently ruins all ability to fight. (several follow-up incidents in different forts agree that it wasn't something else)
*sigh*
BONUS:
That same siege is still marching towards my fort!
Close the gate!
Gate closes, locking the few dwarves that were draining the river outside. Their sacrifice will not be forgotten!
Wait, why are the goblins still pathing somewhere?
They're headed to the drained river? Huh, that's the part right under where my hospit-OH.
*goblin army climbs up through the well*
*facepalm*
Disaster is imminent. Fortunately, the fortress has a backup milita! Unfortunately, all eighteen are novice to competent warriors in semi-shitty gear.
*bloody battle for both sides, with about 1:1 casualties*
Well crap. About twenty goblins survive, and enter the fort!
BONUS, the second:
Well, maybe doberman bomb and peasants with axes can kill enough of them to make them retreat!
*bloody battle for both sides*
The siege is broken! All of my useless dwarves and soldiers and dogs are dead, but the siege is broken! Those last seven goblins will run away!
Oh. Without pump operators draining it, the river has refilled, sealing the well that the goblins entered through. Wait, lets just open the gate to let them out! Oh, right, goblins in front of the lever room.
The goblins are trapped in the fortress.
*facepalm*
*muster every last dwarf in the fortress to kill the stragglers*
*bloody battle x3 combo ensures*
*about a dozen dwarves are left*
Lesson: Cave adaptation is dangerous.