Other mistake: Jack's parents were at a Halloween party when his mother went into labor. Later in the movie when he's asked by his teacher he says his birthday is September 12th.
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Suggested correction: The movie doesn't specifically say this is a Halloween party. This may simply be a costume party which can and do happen multiple times a year.
As you stated, it doesn't say it's a Halloween party. In fact, there's signs in the room that say "Beaux-Arts Ball."
Other mistake: During the graduation scene, as Jack and the boys leave in the car, there is a banner in the background (I think it may have been used when the movie moved on seven years) which says 'seven years later'. Surely it wouldn't be up on a graduation banner?
