Continuity mistake: At the bottom of the stairs there is a picture of Peter and Lois, and the song "So Long, Farwell" starts. The kids start singing and you can see in the backround the picture is gone then comes back when the kids are going back up the stairs.
Continuity mistake: When the children are singing, "So Long, Farewell", as they are going upstairs they pass a picture of Lois' parents hanging along the middle of the stairwell. When they get to the top of the staircase, we see the exact same picture, but now at the head of the stairs.
Continuity mistake: When the black knight appears and says to Peter "What are you doing with my girl!?", there is a pitcher and a basket with a chicken leg on the table. But on the next shot where the camera is facing to Peter, both the pitcher and the basket disappear.