Imagine you are retreating an army. Adjacent there is one empty hex and one friendly hex across a mountain barrier. If your army is mixed flyers and ground units where could it retreat to?
I'd say to either hex, losing the ground units if you cross the mountains. I think this is a house rule, because the retreat rules don't cover this case clearly.

Terah said:

This question has been ruled on before; in the "Rulings from Corey" document I've been maintaining, it's on page 5 of the latest version:

"Q: A player has both flying and non-flying units in a battle and is forced to retreat. The only adjacent friendly area is across an impassible border (mountains or water). There is also an adjacent empty area with a passable boarder. Where can the units retreat?

A: In this case, the player could choose to retreat the flying units (only) to the friendly area, and destroy the non-flying units, or retreat all units to the empty area."

Thanks for enlightening me and for your work on the rulings document.