When Stewie and Brian are in the Doubleheads Universe, before leaving it, they are standing in the front with the door open. You can clearly see a picture of Chris in the back wall with just one head, when he should have two.
During the Disney Universe "Its a wonderful day for pie" musical number, the front door is opened by Stewie and Brian. It stays open up to when Cleveland (drawn like Bambi's Flower) walks in, but then is closed and open by Chris. It can be seen open wide open, but is closed when Herbert (drawn like the queen's old lady disguise from Snow white) offering a red apple.
After Mort walks in, the cast yells "Jew!" and meg (drawn like the little mermaid's Ursula) grabs him with her tentacles. The door is missing.