When recording Lemmings Plus I, I randomly stumbled across an obscure glitch. If a lemming mines through 1 pixel of terrain and said terrain has arrows facing away from the lemming who is mining, the result is the miner will fall through the terrain, but the terrain itself doesn't break. This resulted in one lemmings falling through the 1 pixel while the others were stuck up above.
Link to the glitch in question (the glitch occurs at 30:24)
https://www.youtube.com/watch?v=hyOdggqPyMQ&feature=youtu.be (https://www.youtube.com/watch?v=hyOdggqPyMQ&feature=youtu.be)
This looks very similar to the bug reported by Apjjm in http://www.lemmingsforums.net/index.php?topic=2547.0 (http://www.lemmingsforums.net/index.php?topic=2547.0).
According to the video, you are using V1.43. It should already by fixed in the experimental version V1.47, unless I messed things up and introduced another similar bug.
Yes, this is fixed in the experimental version code. (In fact, you can't even assign a miner in such a situation anymore.)