Goblins attack, and I haven't prepped a military. That's not the dumb part.
Goblin leader, who is riding a giant toad, dies from a serious outbreak of The Stupids when his mount leaps into a pool and drowns him. That's not the dumb part, although it is the funniest.
Goblins mill around rather than moving towards my entrance. That's not the dumb part.
Ad-hoc military is killed to the last man, but manages to whittle down the goblins to just one macegrot. That's not the dumb part.
I dig out the wall of the murky pool the goblin drowned in to reclaim all that lovely goblinite and maybe train and send out a lasher. That's the dumb part. I forgot that murky pools have ramps around the edges, and as a result two consecutive attempts to build a squad to take on that damn goblin maceman ended prematurely when the idiots climbed out of the pool to loot the corpses of their fallen friends and got slaughterised, all before actually training with a weapon or checking my admittedly crap armour stockpiles.
Of course, during this siege I managed to accidentally delete the "home" burrow to which all my civilians were assigned, and as a result there were around fifteen idiots outside standing around and being stupid. Eventually I just formed them into a ragtag wrestler squad and flung them at the goblin, hoping to win through sheer weight of numbers; that...didn't work so hot.
Definitely training up a military earlier in my next fort.