And the winner is:
Under some circumstances you cannot place a room when a roof is the room bounding element beneath your room. I can only assume this is because a roof is expected to be above a room for volumetric calculations. If you adjust the bottom of the wall to sit on the roof it will work or what we did is...
We created an opening in the roof so we could place a floor instead. Problem solved. Technically speaking it is more consistent with the construction methods for a penthouse anyway. It definitely did provide a "what the" opportunity though.
For more depth you can review Erik's blog post.