Now that you mention it, it wasn't a crash, but the only occasion where FPSHell actually discouraged me from playing was in Adventure mode. (I've had "0 FPS", but it was still not unplayable, could have been ~0.5FPS rounded down, and my playing-style in a fortress involves a lot of paused-while-micromanaging...)
This instant was, I assume, due to subterranean disturbances (I never knew for sure, but I assumed cavern-caveins/magma-flow-to-hell caused by mapgen errors in the currently loaded area-scale tiles), although I'd also been setting up campfires as it was shortly after their introduction as a bogieman thing, so maybe excessive fires (NB: not wildfires, that I know of, anyway) or just-beyond-sight bogieman armies trying to stalk me. Whatever it was, I remember that stepping one tile took something like minutes for the game to respond to (so measurable somewhere in the range of microFPS, I would say), and I thus slowly (play-time, not game-time) wandered back and forth trying to map out edges in the effect (and light campfires as part of that, ironically, but perhaps helping prove that no surface fire/temperature issues were actually involved).
My memory is hazy about the details (I think I remember noting it in a thread, that will contain more accurate impressions and facts straight from the incident) but I'm sure I just tired of trying to tie it down, and/or a version update came along and I perma-archived the prior worldgens (Adv/Fort) never more to interact with them. I'll probably still have the DFvX.YZ directory, somewhere, but it might easily be on a long-unplugged HDD, even if I track down what era I'd need to look at.
And, as mentioned, it never crashed, which probably makes this a useless (as well as vague) reminiscence to recite. But recite it I did, I suppose!