Revealing mistake: When Belle exits her house, the reflection on the window by the door shows a rectangular flat roofed building, nothing to do with the old gable roof we see in further shoots. In the making-of we are revealed that the prop building is in fact rectangular and flat, with a green screen on top to allow for the CGI.
Revealing mistake: A "green screen" was obviously used at the restaurant (all-you-can-eat buffet). For example, the people supposedly seated in the booth behind Greg's mom were sometimes literally back-to-back with her - there was no back of the booth between them! They were also larger (closer).