I just did this one late last night...
I'm preparing to tap the underground river I have... it's actually located 8 floors up above "ground" level compared to the rest of the fort, and I have a bottomless pit, so my first order of business was to make a giant "water tower", a 6x6 vertical tube 23 z tiles tall to store (and pressurize, it's halfway across the map from everything I want to connect water to) the water, and a channel along the lowest floor leading to the bottomless chasm as a "drain" I can link to in case of emergencies. Except, instead of just being 6x6, I took off the corner tiles, so that it looks somewhat "rounded". So, I dig out three of the "walls" off the start, digging channels back towards the staircase, where I will wall the place off.
I start digging, but then forget that support/cave-ins don't include diagonal support while designating channeling.
CAVE IN! It flings my worker backward, and she's dazed because she was standing near the cave in, but she isn't crushed... except it flings her into the channel I already dug... about 19 z levels straight down to the bottom, where only 23 stones are there to break her fall.
Incidentally, her head actually bounced an entire floor up, while her arms and legs stayed in the same tile.
You'd think I would have figured out how not to cause cave-ins by now... and she was really close to hitting legendary, too, so it's going to be a pain to train replacements, especially since I've been uncovering all these ore veins, but holding off on digging them out until more of my workers were at "master" level or higher.