It's truly amazing how such small mistakes can snowball completely out of control.
For some reason, my goblin sieges always seem to start just as my caravans are are getting in, and this was no exception. Normally I don't care, the humans and elves don't have anything I need, but this was a dwarven caravan carrying much needed flux stones, as well as coal and lignite (I have magma forges, but I don't like cutting down trees to make fuel).
I have designed my main entrance so that there is a passage from the outside to the inside that I can seal on each end with drawbridges, and the trade depot is off to the side of this passage, separated by retractable bridge. Above is the standard chamber of magma with multiple hatches to dump a large quantity of magma onto the goblin hordes.
Once the first squad of gobbos get into the chamber, I realize they are armed with bows and decide to leave the retractable bridge to the depot open, thinking maybe they could defend themselves. They didn't. Anyway, the rest of the goblins and trolls wander in slowly, so I close raise the outer bridge...forgetting about the depot bridge. Magma quickly floods the depot, ignites the coal, and begins melting my flux and all the other items inside. This was unfortunate, but I made a mistake, oh well. I forbid all the burning item and empty the floodzone and start cleaning up the mess that's left. Everything is back to normal.
ELVES
Spring rolls around and that brings the elven caravan. Meh, they have nothing I want, just wood and clothing, don't need any of that, so I can just ignore them. Can anyone see where this is going?
I get "unpacking goods" message and suddenly remember the molten metal and burning coal. The trade depot is filled with more smoke than normal, and a purple gas that turns out to be boiling wine. The elves quickly pack up, but not before some die and some bins break open. My genius dwarves decide to run in and claim items, in the process receiving serious burns (which I don't notice), a couple minutes later I get a "Urist McDead has bled to death" followed by several more. This tantrum spiral has lasted 3 seasons, dropped me from 154 dwarves to 71, and has only just now started to level out.
All of this because I left a bridge unretracted.
I have subsequently redesigned my entrance to use a draw bridge to protect merchants from ranged attacks