Also doable with other skills, but miner is what I discovered this with (while testing to see if a certain other bug happens - the other bug I suspected did not actually happen).
See attached level for an example - assign the miner towards the right just before hitting the wall, with the first lemming (there's steel placed to prevent assigning too early for the correct position to trigger this).
After some examination of the code, I've determined the reason for this - different skills may move the lemming downwards by different amounts before transitioning to a faller, ranging from 0 to 3 pixels. This was the cause of the fall distance difference between walkers vs bashers/miners in DOS, but NL has long since fixed that side of things - splat distance is consistent. Then, the lemming pulls out the glider - theoretically - after falling 7 pixels. Even for this - NL correctly tracks it regardless of the downwards movement at the start.
The bug arises from the fact that a faller falls 3 pixels per frame, but the "do I become a glider now?" check only happens once per frame (before any downwards movement). If the downwards movement before transitioning to faller is 0 or 3, then the lemming will fall a total of 9 pixels before transitioning (as on the first frame it will have fallen 0, the second frame 3, the third frame 6 - none of which are enough to start gliding yet); whereas if it's 1 or 2, they'll only fall 7 or 8 pixels respectively (1, 4, 7 or 2, 5, 8 respectively).
This bug also affects floaters, although the impact there is usually far more subtle - purely a matter of "the lemming might hit the ground one frame earlier or later when the bug is fixed".