Revealing mistake: Before Josh tells his mom about Buddy, Buddy starts to push the basketball with nose down the stairs. While he is doing this, you can see the treat put there just before the ball moves.
Revealing mistake: Towards the end of the film when the Yankees are trying to close out Game 6 of the World Series you can tell the "real" footage they use is not from the World Series. They are playing the Atlanta Braves in the World Series but you can see the Baltimore Orioles logo on the field in a number of shots.