I’ve been on the Whitney main trail about 15 times, and have occasionally lost the trail near Mirror Lake. But the terrain is pretty mellow there, so I’m not understanding how a long fall is likely. I’m sure that there some drop-offs a few hundred feet away, but it seems like they would be easy to avoid, even in the dark.
There is a chute down the cliff side that someone might try to descend (from the white arrow switchback). Combine that possibility with Hobbes's post about how people tend to panic when lost in the dark, and it all adds up, starting with losing the trail.