Arthur makes a great deal about finally having a window in the basement, but in earlier episodes there is a window at the bottom of the stairs and another in the wall opposite Arthur's bed.
Arthur finds a window in his basement room, and from the outside it is to the right of the steps. There is no room for a window to the right of the steps in all of the establishing shots.
When Arthur is showing Holly the new window, the box of Nilla wafers changes position on the table between shots.
Bedding salesman says Doug and Carrie can save $200 buying the identical set without all the Danish stuff. Tempurpedic is a Swedish brand, not Danish.
There is mention of the front door being in the middle or on the side. In the 5th season's finale, Doug and Carrie are on their way to dinner and exit the front of the house. The door is in the middle with windows on either side.
Arthur's window is mounted in the frame backward. The working parts of the window are on the outside. This would never be the case if for no other reason. Security.
When Arthur is watching Lou Ferrigno out of the basement window talking with various people the backdrop is an obvious painting.