Okay, this happened to my wife twice on Classic. When she was on a flight path and Alt+Tab’d to check something the gryphon disappeared and she fell down.
I’ve never seen anything like this back then in vanilla, and while there is a variation of this bug in the current game (where, if you join an arena while on a gryphon, sometimes when leaving, the gryphon doesn’t spawn and you fall down), there it doesn’t matter as much.
However, Classic has several places which are meant to be inaccessible, and only seen from above like the Shatterspear Valley between Moonglade and Darkshore. Since this bug happened twice already, it might be easy to trigger intentionally and fall down to these places.
Blizzard might want to fix this soon. Is it well-known?