I have some "fun" ones. The first I really should have seen coming...
I finally got a map to the point where I've got access to all the basics of awesome. Magma, good farms growing things above- and below-ground, specialized housing for farmers and clotheirs, a very fancy place for my incredible gemcutters, and virtually no combat training for my dwarves. What can I say, I haven't seen anything break down walls or ceilings yet, so my entire invasion defense plan was "QUICK! GET EVERYONE INSIDE AND WALL UP THE ENTRANCE!" It worked quite well for a year or two, and then I got cocky. I built a zig-zag dog run at each entrance, and used all that free copper to just populate about 100-125 cage traps. This seemed to go quite swimmingly... because now I've basically got a bunch of goblins and trolls showing up on a regular basis to bring my dwarves MOAR FREE STUFF! Anyone who made it through the cage traps generally didn't fare very well against the fully-loaded-with-silver weapon traps. I was not satisfied with my unassailable "Goblin Equipment Donation Outpost" even though I could just sell it all off to passing caravans quite easily... No... I had to get nasty about it.
So I built a nice, high tower--having recently experimented with some 8-level deep pitting over a training room, filled with eager, hardy dwarves ready to beat some goblin faces in with wooden weapons, this seemed like a good plan. Only this time, I made the tower 15-levels high, so there would be zero chance of survival. I did this by building 12-levels up, and then digging three levels down so there would be walls to catch most of the loot so it didn't go all over the place. Goblins were caught in the traps, the dwarves would dutifully carry them to the top of the tower in my Room Of Strippery Judgment. Once they were carefully stripped of everything of value, I'd pit them to their deaths. This went on for awhile, and then I got lazy. I decided to stop stripping them first and just let the dwarves take whatever wasn't too gore-soaked.
I should have taken the hint from the body parts that were now scattered all around the outside of the pit. I really should have. I finally unlocked the doors at the base of the pit and let the dwarves in. There was a mad rush to collect all the gear, and I started having some miners double my loot storage space. Once they'd done with that, I noticed dwarves were still rushing in there to grab something. I'm up near 240 dwarves at this point, so it's just a hive of frantic activity and I don't look too closely.
...then I noticed my refuse pile was full. Hrmm... Assigned a new one, watched dwarves fill it with body parts, one per square. So I made another large refuse pile. They filled it up, too. ...and another, and another. I'm sure some people would consider this setup to still be perfectly viable, but I wound up abandoning the fortress after roughly three-quarters of the landscape was covered in body parts. Body parts literally all around the fortress, and I can't imaging why traders didn't just turn around and run like mad.
* * *
My most recent facepalm was ignoring the new "Urist McUseless was found dead... drained of all blood!" messages. I just figured one of the little idiots had cut themselves, went to their room alone, and bled out. It is at this point I'll mention that I have a very aggressive condominum program, where until I've gotten two floors done with about 100 rooms apiece I don't stop building new rooms so that every dwarf who can leg it over gets their very own 2x2 room with polished floors, a bed, and a cabinet. Every. Dwarf. Sleeping. Alone.
I also prefer to just suck it up on the lag, and rig a 4 piece staircase running through a central vertical shaft, around which I build everything, and then in this 3x3 area with statues in the center, I cut out the rest of the tiles, put down metal bars, carefully construct a waterway exit at the bottom going to the edge of the map, and at the surface with a few zig-zags to decrease flow below what the single exit channel can take, run a conduit to the nearest river so that water flows down in waterfalls right through the center of everything and out the fortress, eliminating contagion problems, a number of issues with forgotten beast blood-related maladies, and making sure the dwarves are more or less deliriously happy (although I don't know how they tolerated the smell of 100+ perpetually dampened dwarves).
Apparently this combination of factors, left unattended long enough, has interesting problems of it's own. Since the VAMPIRE--who I knew nothing about the existence of yet--came in at about wave three and looked like he could maybe handle a weapon so he was assigned to my first squad of goblin squashers. Very early on I screw up and some goblins get in... A merry slaughter occurs, you guessed it, right in that central staircase/water-park. Most of my dwarves make it so I figure, no big deal. I continue on with the building of individual rooms, farms, and now... mausoleums.
It takes awhile for me to realize it, but when I was staring at a screenful of 14-seater mausoleums, in a fortress of about 87 adult dwarves (down from 145 with me struggling to keep their numbers up the whole time) with "only" 125 rooms to sleep in, many going unclaimed, I thought "Hmm... I haven't had that many goblin incursions." Now the curious messages about dwarves drained of blood has me thinking maybe I'm missing something. Perhaps some new disease that causes dwarves to randomly bleed out, like ebola or something. ...or... "Haha there's a vampire eatin' 'em all." Did I mention that I had over 100 children in the fortress by now?
One Google search later, I discover that Toady One has indeed added vampires to the game. Oh dear. I read the wiki about the various ways of detecting them, particularly the bit about the long personal history. Now the dessicated dwarves and all the children start to make sense. Lo and behold, my manager was a vampire, by virtue of being the only dwarf with the job who didn't up and die on me. I decide it's time for him to pay his debt, so I build a small bedroom with a token, undecorated bed, and a small dining room connected to his office, schedule a few jobs, wait for him to show up for work, remove the door and slam some iron bars into the doorway. A few minutes later I see another spate of drained dwarves messages. I ignore it, figuring I was too slow to notice them beforehand. Still later, the messages keep coming. Okay so maybe I have two vampires somehow... oh were I that lucky. I started looking at the other dwarves very carefully because I was having trouble finding people in Dwarf Therapist so I could remove all inessential jobs from the new manager (since the vampire manager apparently got fed up and decided his office was no longer his office and stopped issuing work orders). To my surprise, he appeared to also be a vampire. Okay so I'll just seal him in, too. No problem, right? WRONG.
Apparently, during the initial skirmish, vampire blood from my vampire hammerdwarf made it's way into the waterfalls and must have gotten into the waterskins or something. Every time some noble died, I'd just replace them with another reasonably fit dwarf. So... not only my manager was a vampire, so was my bookkeeper. ...and my broker. ...and my chief medical dwarf (LOL). My Captain of the Guard? Also a vampire. My baroness? Vampire. Militia captain (mainly an honorary title since I never made a second battle squad)... vampire. Every time I'd replace a noble who'd been vampire-kibble, the ones that didn't die and vacate the position appear to have been doing so because they were also vampires who never slept and thusly never get caught unawares in all those nice single-occupancy rooms I'd made.
I gotta hand it to Toady One... that's the first fortress I've seen that was completely f**ked before I even had an idea something was wrong. I'm not stupid enough to try reclaiming that. At least when I cut into the mysterious adamantine-vein-nowhere-near-lava right next to my new magma-powered "Deeprun" factory and dorm area, I had an idea something might be about to go wrong before I Dug Too Deep at about -14. (Did you know those things will actually eat your furniture? I do now.)