This goof is in reference to the design of the Wilsons' house. Throughout the series, on the outside of the house, there are two windows on each side of the front door. On the inside of the house, there is no window by the front door. The window is around the corner and to the left, and it is a little bigger than the one by the door as seen from the outside of the house. The house design on the inside doesn't match up to the design on the outside.
Dennis: Heeelllloooo Mr. Wilson!
Sylvia Field, Irene Tedrow, and Billy Booth make their first appearances in this episode.