When the white cat exits the can to get the Little's attention, he is last seen at the rear wheel of the taxi, but at the next shot, he is back at the can's opening.
Falcon's wingspan is noticeably shorter in the "Going up!" scene in which he carries Stuart's plane higher into the air.
Snowbell runs up to the back door of the cab trying to get the Littles' attention. However, when the cab moves away from him, he is now standing directly in front of it.
When the family is riding in the taxi chasing after Stuart, the baby girl is securely strapped into an infant carrier. When they climb out of the taxi, the baby girl is in the mother's arms and the infant carrier is nowhere to be seen.
When Stuart confronts Falcon in the Pishkin Building, Stuart and Margalo are alternately standing on and off of the grillwork surrounding the water valve on which Falcon is perched.
When Falcon is arguing with Margalo, they are atop the Chrysler Building according to the eagle decals, but when they swoop down, they are next to Central Park. The two are nowhere near each other.
They are actually on top of the Pishkin Building, which is fictional and could be located anywhere.
They are actually on top of the Pishkin Building, which is fictional and could be located anywhere.
In the final shot of the movie, as the family leave the rooftop, Mum is holding out her hand as though she is carrying Stuart, but he is not there.
Toward the beginning of the film when Stuart flies the model plane outside and crashes it, his mother, father, brother, and friend all run out the house and into the park to chase after him. This means the baby was, presumably, left alone in the house - or, the writers just forgot about her completely.
Mrs. Little refers to Crenshaw as Mr. Little's uncle at the beginning of the film but Crenshaw is his brother.
When Stuart calls home on the pay phone, he is speaking into the wrong end of the receiver.