It's possible on this map to go under the map from one specific location.
http://z1.zod.fr/z/terrain08d-ztj.pngAs you can see, I've got a soldier under the map. He enter in this bug map by walking from the location of my female assault rifle soldier to his actual location.
It would be a minor bug, unless an alien's going all time under the map, where it's impossible to kill him. I'm retrying the mission for the 6th time...