Near the end of the movie, when he hurts his arm and loses the 98MPH fastball. The MLB rule for the hidden ball trick is that the pitcher can't be standing on the mound when it happens. In the movie he's standing on the mound.
I am a big fan of baseball, and I think you are allowed on the mound, just not the rubber. Good pick up though. If you hadn't of added this one, I would have.
In response to the first comment: The rule states that you cannot be on the mound. Another rule the movie breaks is "The Floater". It is illegal to throw the ball under handed. Even though some pitchers are "submarine" pitchers and come close with their side-arm deliveries, "The Floater" would never be allowed in any stuctured baseball game.
Actually, there used to be pitch that was used in major league games way back when players didn't take 'roid called an ephus. It very much resembled the high slow pitch soft-ballesque pitch Henry used. It was a legal pitch and you don't know jack.