Revealing mistake: Mr. & Mrs. Castle wish for a million dollars in 5 and 10 dollar bills. But the bills they receive all have the same pattern printed on both sides, a pattern which which doesn't resemble any real US currency. There's also not nearly enough of the bills to total a million dollars.
Revealing mistake: Make-up for the space aliens was apparently changed after the closing scene in the store was shot and before any footage of the aliens' "true appearance" was actually taken. This resulted in Frisby, who is now telling the truth and finding no one to believe him, saying that the aliens had a single eye in their foreheads, when in fact, the aliens each had two eyes, in the traditional location.