My fort was coming along nicely. I had just completed a large moat + wall project, covering most of my 3x3 embark in a temperate zone. My river froze every winter, but only by the barest of margins. I was too slow the first winter, so for the second I decided to freeze (ha!) my miner's other duties and focus on quickly getting some floodgates set up.
I had barely dug out the total of 10~ ice squares, with half of the floodgates delivered, when the river unfroze. I knew it was quick, but not THAT quick. Sure enough, "Urist McFarmer has drowned" splashed up. Fortunately not a legendary miner, I thought, as I double checked the alert log. Turns out, FIVE dwarves drowned... I should really restrict dwarves from moving around on my (large) frozen moat.
Now, I've only got the barest of essentials. My dwarves are holed up in the soil layer in tiny, hastily constructed temporary rooms that were barely adequate for my original seven. I was now at 63 dwarves... uhh... 58 dwarves, and the tiny 6x4 dining room was perpetually packed with dwarves and dogs. Taking a brief glance down there, I noticed that almost all my dwarves had red !'s above their heads due to the lack of... well, anything good in their lives. Normally they'd be riding on content but I guess the cramped quarters + the death of five friends really pushed some over the edge. Tantrums started immediately, followed by berserks in short (ha!) order. Fortunately, I formed a functional military very early in my game who are pretty much in their own clique and able to "respond" to angry dwarves rather quickly.
The interesting thing, though, is that the berserk dwarves aren't the biggest pain; the tantrum dwarves are. It's temporary, but that just means that my military won't go smashing their faces in. My two legendary miners, armed with steel picks, have been carving dwarves up left right and center.
Reinforcing my assertion that tantrum->berserk, Legendary Miner #1 just went berserk. Aside from hacking off a militia captain's arm, she was smashed to pulp in short order. Prior to this she had personally killed over five dwarves during tantrums. My only other miner has a completely shattered leg from tantrum-picking a hammerdwarf. Five of seven founders are dead with the final two miserable.
Down to 26 dwarves (from 63!). An injection of 21 migrants just arrived. We shall call this event... Winter's End. Every year the survivors will enter a 3/7 water pit and beat each other into a pulp in rememberance.