Revealing mistake: When in Boston, while walking through the park, Susan and her mother are obviously walking on a treadmill because the park background is moving way too fast for their walking speed.
Revealing mistake: In the taxi scene at the end of the movie, the steering wheel turns back and forth but the driver's hands are not moving.
Revealing mistake: At the wine tasting, Lily pushes the waiter. It is obvious that the glasses on his tray are stuck to it, as they don't move even when he turns the tray on it's side.