If your BuildingEd screen often looks like this, and you don't know why - make sure you select the correct layers often. The layer you've got selected (and checked or unchecked) will impact the way your tiles render as you create your building.
Hey its me again, this time with a bedsheet..
How to reproduce:
Place a two stacked boxes on top of another.
Now, pick up a bedsheet, and place it on the window from the inside.
You should get something like this: