Defenestrating sphere

[Braininthejar]

What happens if the sphere hits someone who is already prone?

[Arturick]

Amusing ragdoll effects when they get tossed back up in the air.

Seriously, though, the spell renders them prone when they land.  It doesn't say anything about them having to not be prone in the first place.

[Kethrian]

Skip the step to knock them prone.

[Halinn]

They get tossed out of the nearest window. Regardless of how long away that is.