If anyone wants to bother worldbuildering it (yeah, I'm lazy), there's a semi-related bug that let's you stack units.
U|F|M
U - your unit, with at least 3 movement
F - friendly unit on a hill
M - mountain, still covered in fog because of the hill under F
Right click on M (covered by fog) and your unit will try to move over F to land on M, but will be unable, and will stack with F.