I was making a main drainage system for my reservoir in case I needed to drain it to widen the intake or whatever. Instead of channeling each Z-level at a time for the long vertical 2x2 pipe, I just chose to channel the entire thing at once, leading to a lot of cave-ins and a couple broken bones, but no biggie. I expected it.
Fast forward ten years when a whole lot of goblins and forgotten beasts decide to attack. I had (supposedly) sealed off the caverns from the three or so forgotten beasts that festered there because I didn't have the strength after my military was attacked by one which fired fireballs (which caused their booze rations to catch on fire and explode, causing what is akin to a hand grenade malfunction)
My fortress is halved in population by now, and after (barely) surviving a goblin siege, I look in my fortress and see webs everywhere.
"Wait what"
A forgotten spiderbeast had climbed up through the drain, which emptied into the caverns (with a maintenance staircase no less!) because of the imperfections making it essentially a very curvy system of ramps. Facepalm moment one. I killed it off and yadda yadda, fortress was dwindling.
ANOTHER SIEGE. This one with a whole lotta trolls! My elite archers managed to pick most off, and I actually got the drawbridges up before anything really bad happened! Then I noticed my emergency mayoral escape tunnel's exit's door was missing. And it's hatch. Facepalm #2, totally forgetting Trolls could destroy anything they damn well pleased. I actually fought them off, though!
Built walls in place of those doors in that emergency mayor thingy, and placed a pickaxe in my mayor's office so if s/he needed it to break open onto the outside world, s/he could. Handy-dandy. All of the forgotten beasts were at bay by destroying some important staircases, as well. Everything was fine.
Cut to another siege. Bridges up, everything okay. Trolls get in. How? A drawbridge that I used as the door to our waterwheel generator, which pretty much led into our pump stack. Cue goblin siege killing everyone, because I didn't think of putting a moat there. Facepalm 3. Brainpath the Metropolis was now a smoldering heap of dead bodies. All of this happened in one or two years.