Beverly Lynne and Lizzy Borden have their names misspelled in the credits as "Beverly Lynn" and "Lizzie Borden" respectively.
The cartoon swirl effect (that implies that they're trapped in the cartoon dimension) is clearly visible through the windows of the doors yet the characters act surprised when they open the doors and see it.
Obvious fake plastic pizza that the Shanghai Pizza delivery boy brings to the house.
Dr. Carnage folds the pizza into his mouth to make it look like he's eating it but it can be clearly seen that the pieces of the pizza are simply falling to the floor.
A cartoon cop exits Candy's TV in her room and is then met with Cindy and Amy. The film doesn't explain how they got there as it would require them to walk past the living room where Dr. Carnage and Max Assassin are to reach her room.