Revealing mistake: The hallway that leads to the den and the back door of the kitchen has a door at the end of it, when this door is opened we see what looks like pine trees and mountains, we know that the Stevens have neighbors on that side of the house, so that when the door is opened you can blatantly tell that it is a picture.
Revealing mistake: In several episodes, the window over the kitchen sink is shown from the outside, while snoopy people look in. The window shown in these exterior shots is far too low to be the same window that is over the Steven's kitchen sink, which is shown in many shots looking out.Super Grover
Continuity mistake: In the early shows, the door between the dining room and kitchen is a swinging door with louvers. In later shows there is a regular sized door with partial louvers. However, after this change, there are a couple of episodes within the same season, where the regular sized louver door is a solid door with no louvers.Super Grover