What happens when you try to hide a unit or move a hidden unit to a location where you already control a hidden unit, given that each Facedown Zone has a maximum occupancy of one card?

Ruling: It becomes an illegal action/target, and that part of the effect doesn't happen. If there is targeting involved, you could not choose that battlefield to begin with.

Nuances:

• This applies to effects that would hide a unit at a location where you already have a hidden unit
• This also applies to moving a hidden unit to a location where you already control a hidden unit