[h3]Incorrect Normal Mapping[/h3]
Normal mapping was somehow incorrectly implemented on UFO interior walls.
Strange normal mapping artifacts appeared where they were not supposed to on some tiles as in this image:

I was able to identify what I believe to be the root cause, and so now this should appear correctly:

[h3]Aliens shooting through walls[/h3]
Players would be getting shot and taking damage when no enemies were visible. It turned out to be the aliens, not the robots. I made sure they would no longer be able to do this. Several play-throughs have affirmed that it's working, but let me know if it happens again.