Climbing multiple ladders (=floor openings) in succession can cause one to fall to the ground floor, occasionally breaking a bone or two.
Reproducing the bug is very simple; take a tower consisting of floor openings and start climbing the ladders up. Moving (WASD) is not required.
I have seemed it easy to reproduce by means of the default action (F) though it would surprise me if it does not occur with the traditional right click -> climb up, or by a manual keybind to another key. The procedure requires quite accurate timing, though with enough practice you will be able to suicide an alt in under a minute, see the video below:
https://i.imgur.com/gg1qdin.mp4
Interestingly the corpse spawns on the third floor:
Note: Damage does not occur on every tile for some reason, but is common enough to be slightly aggravating.