Current fort - While filling my magma reservoir (it covers 3 z levels, and is 40x40) I left the pumps on, and part of the pumps supplying a smaller reservoir overfilled, causing my entire powertrain and 2 z levels of farms and tree farms to catch fire. Oops.
Older fort - a hill titan attacks (beware its fire). I send out the military to defeat it... resulting in the surface of the map catching fire, obliterating almost the whole military (and everything above ground). It invades the soil layers, where the farms are (as well as food and drink storage) catching a few dwarves unaware. That level burns. The last !!military!! member, who has had all the fat melted off him, and is near death, manages to decapitate the titan. After the fires die out, nothing is left but his armor and weapon, which are placed in his tomb (platinum sarcophagus, slabs dedicated to him and the hill titan, more lavish than any king would ever deserve).
Mostly, death by fire. I've had no recent accidents with an aquifer, just tons of !!Fun!!