My first megabeast (a titan) busted into my fortress, killed about 10 or so dwarves and a few war dogs, got caught in a cage trap, got released, killed a peasant, and got hacked apart by my entire surviving military.
One of the first dwarves the titan killed was a marksdwarf with five children. The marksdwarve's current helpless baby died of thirst, and one of it's siblings committed suicide shortly after.
Another marksdwarf got post traumatic stress disorder and kept being "Interrupted by Titan" every single turn, even when the titan was gone. Eventually, I killed the dwarf with a wooden spike to put it out of it's misery.
Coincidentally, the titan was released and killed during several simultaneous ambushes. About 30 goblins swarmed a few of my miners and a pump operator and killed all of them. At the time, they were trying to damn a river that I had accidentally started flooding the valley with. Of course, the river did not get dammed in time, and now it would be nearly impossible to successfully dam it. My entire military was tied up at the time, so I couldn't do anything about the goblins.
Currently, I have a wall of water surging towards my biggest food stockpile's entrance. Unfortunately, my bedroom complex shares this entrance.
Ugh.