Not a fatal mistake, but still a facepalm of sorts.
Anyhow, I haven't played DF for a very long time, and even at those times I played it sparingly, but my current fort was going relatively well despite this handicap. They hadn't died of thirst, been killed by zombies, or eaten each other's ears yet, so I was doing very well. However, I started to get a little ahead of myself. I was making loads and loads of masterwork goods, everything from bolts to roasts. And, of course, this meant that a goblin siege would be inevitable.
It took some time, but it came. Before that pivotal event, I had managed to double my military from five marksdwarves to ten, and had procured one master marksdwarf. My resolve was reinforced by the fact that the goblins had neglected to bring archers of their own, although I do admit that the giant lizard their leader was riding looked frightening. I immediately raised the alarm (or as I called it, "Code Kimberlite") and my dwarves came rushing in. So far, the only problem I experienced was the fact that my soldiers were unwilling to raise the drawbridge, but some quick fiddling with the burrow allowed a metalworker to fulfill that crucial role.
With the fort sealed, I parked my squad behind the fortifications, eagerly awaiting the coming slaughter. At last, the enemy came rushing up the hill, and my marksdwarves let loose a tremendous volley. These shots were either blocked, dodged, or missed entirely, but I had plenty of ammunition and an overabundance of confidence. However, I felt myself falter a little when I noticed that the goblins were continuing up the hill, past my main entrance. I had never dug any other entrances to the main fort, so what could they be doing? Were one of my dwarves still trapped outside?
And then I noticed.
It turns out that in my infinite wisdom, I never gave a thought to the more undesirable properties of windows. In particular, their smashability. Now, as it happened, I had installed a two tile long window in my bookkeeper's penthouse, because I decided his personal art gallery needed some light. Just as it began to dawn on me, the goblins had proceeded to smash down that very window. I panicked, but thankfully my momentary indecisiveness did not plague my fort as I had it paused. I ordered my squad to the hallway below the penthouse, and hoped against hope that they could pull something off.
And what'd you know, they did! Their leader, still atop his reptilian mount, was the first to fall, and one after another his cohorts were turned into glorified pincushions. Only a few of my dwarves suffered any injuries, but nothing that has proved fatal. I can only guess that the single stairwell that led to the penthouse had funneled the goblins into a neat little line, presenting the perfect targets for my men dwarves.
So, I made a mistake but managed to fix it (kind of) and managed to learn something important too. Today was a good-ish day.