I had multiple facepalms streaming from the same incident over the last couple days; it all started with my first dragon showed up on the map. He ran around torching a kitten who had wandered out into the forest, burning down most of the vegetation in that vicinity, until the kitten was no more and then he pathed towards my fortress and into a cage trap.
I've got a pretty reasonable walled river setup, and a mass-pit drowning chamber adjacent to the river had been my main source of goblin disposal. Now that I had a dragon (who had just demonstrated his awesome firebreath to me) I decided to drain and deepen the drowning champer several z-levels, create a 3x3 block of fortifications (with the center block removed) and drop the dragon into the middle. My thought was firebreath is more painful than drowning and also burns the refuse, so I no longer need to drain and refill my chamber to prevent a heinous buildup of trash+miasma.
Facepalm #1: Dragonfire is really, really easy to block. Only the weak fleshy vermin that wander into traps (and the occassional 'misplaced' kitten) ever die by fire in my pits. Now I've got a dragon thoroughly protected that I don't know how to get rid of (short of having marksdwarves with the ability to shoot through fortifications from range.)
My new solution: carve fortifications into the area around the death pit, so my marksdwarves can practice shooting goblins and the dragon can burn up all the fallen wooden bolts. Eventually they should get talented enough to shoot through fortifications from range. Bonus points because the markswarves randomly get hit by dragonfire (which they also block and train up shield skill.) Previously my marksdwarves were spending every other month on training (danger room) and off duty (for archery targets.)
Facepalm #2: Markswarves carry alcohol. When they go off duty, they turn into civillians, and drop their alcohol+food where they stand. Even if that's next to a fortification receiving regular bouts of dragonfire. With a little unfortunate timing, alcohol boils and explodes, thus killing the would-be Marksdwarves.
In further retrospect, I believe I can send my marksdwarves directly into the death room and just have them shoot the dragon from two square away; they should block the dragonfire long enough to take her out. Alternatively, I should be able to drop a bunch of stones through the dragon's entry pit onto her head until there's nothing left but a big pile of dragon goo, then deconstruct the forts. I am anticipating some Facepalm #3 as a result.