Factual error: Arthur when driving the car mentions speed cameras, which wouldn't have existed in 1960 when the film is set.
Continuity mistake: The toy Ferrari almost gets run over but ends up in front of, but between, the car's two tires. When the man returns to his car, the toy car is on the outside of the front left tire.
Continuity mistake: Arthur had only 36 hours to spend in the Minimoy world, and he leaves at midnight. He spends a full 24 hours too long as a Minimoy. After he finds the Minimoy Princess they travel during the day. They spend the first night sleeping in a flower (24 hour mark); the second night he is wandering under the house, the grandma is sleeping in a chair above him (48 hour mark). He then returns at noon. That's 60 hours.
Factual error: When Arthur is in the bar, the music is chronologically out of order. The film is set in the fifties. One of the songs playing is Staying Alive from the seventies.