UFO 2.3 Developer's Edition R23665 (With Complete Maps Rebuilt)
Seen this happen on BRIDGE map of the same build to both npcs on the bridge and my own guys on the hill leading to the bridge on the left.
In the former case, aliens could be seen, but they "float" in air under the bridge and were practically invincible.
As for my guys, the just "fell throught the ground", and could apparently move within a small area, only their green highlights being visible, and could not be walked to any visible location:
http://s42.radikal.ru/i097/0903/a9/ae90db718a0e.jpgOn same map, having brough my soldier up on the hill near LZ, could nopt move him down any more, that's the place he got stuck:
http://s54.radikal.ru/i145/0903/4d/07c3af394896.jpgOn one of the base maps, sorry can't recall which, (1) no texture on the ground, and (2) alien got stuck facing away in the corner, not moving anywhere. He turned to attack me when I got closer OK, but prior to my walking a soldier to withing just a few tiles of him he stood there and did exactly nothing:
http://i039.radikal.ru/0903/71/462f21d37497.jpgApparently the bug is not dependent on WIN version as mine is XP.