The stamp dealer who sells Monty the rare "Inverted Jenny" stamp claims that "of the one hundred of these stamps originally printed, this is the only known copy in existence." In fact at least 90 specimens of the "Inverted Jenny" are known to survive in the hands of collectors.
In the courtroom scene the judge says, "So you were making advances to a woman who was clearly involved with another man," however, in Monty and Spike's previous statement just before she said that, they both clearly stated that they did not know the woman had a fiance. Even though their statement was untrue, the judge did not know that at that moment.
On the cover of USA Today it shows a picture of Monty Brewster in front of a podium with his "None of the Above" slogan. The article was to highlight the Yankees exhibition game, and before Brewster had decided to start his mayoral campaign.
In the Yankees game, Ken Dixon's first at bat is as a left handed hitter. But when he hits the grand slam off of Monty, the TV shows a right handed batter hitting the home run. He could be a switch hitter, but he wouldn't face the same pitcher from both sides of the plate.
It is often claimed that by using the rare postage stamp to mail a postcard Monty violated the clause of the will forbidding destruction of inherently valuable property. However, as Monty was using the stamp for its originally intended purpose this would not go against the terms of the will.
In the opening baseball scene when Brewster is facing Rudy, he is wearing a batting helmet with ear covers over both ears indicating he is a switch hitter. When facing Brewster, a right handed pitcher, he is batting right handed instead of left handed which would have been to his advantage. However, as with even lower levels of play, in the minor leagues there is certainly not always the luxury of having a single-flapped helmet for each batter and often non-switch-hitters end up wearing two-flap helmets as well since those type are often the ones carried most by teams since they can be used for right-handed or left-handed batters.
Toward the end of the movie in the boardroom, when Warren is talking and says "a couple of rabbits" the bald side of his forehead is on the left. When he talks again, you can see the bald side of his forehead on the right flipped around. The next shot of Warren it returns back to normal. It takes place all within a few seconds.