Not sure if it's a Mod bug or a Fireaxis Bug. It happened to me when:

Enemy: flying drone
Soldier: Ranger
Weapon: Tier 2 Shotgun
Enemy Tile: Same (X,Y) of Soldier but 1 Tile above (Z).
How: The drone didn't initially see the soldier and, during his moving turn, it moved to the tile above the soldier (or MAYBE exactly the same soldier tile X,Y,Z accidentally because it didn't know the soldier position and was probably going to a tile it considered empty... I'm not sure)

Please fix! Thanks