1) Charlie Sheen had pitched in HS and was so amped for the role that he took steroids to keep his arm healthy and was hitting 85 MPH on the gun during filming. They shortened the distance from the plate to the mound from 60 feet to 50 feet to make his stuff look that much faster.
2) Dennis Haysbert really hit the home run vs. the Yankees in the movie. The actual homer hit the top of the wall and bounced into the stands at Milwaukee County Stadium with 20,000 extras going nuts. Haysbert was so emotionally overwhelmed that he didn't realize he hadn't dropped the bat as he ran around the bases.
3) Corbin Bersen accidentally really hit Charlie Sheen in the face when Dorn sucker punches Vaughn for sleeping with his wife.
4) By far the best; Wesley Snipes was so slow that they used slow-mo on almost every single scene where he's running in the movie, and he was so bad at throwing the ball that you never see Willie Mays Hayes throw the ball back into the infield.
5) Jeremy Piven was in the movie as a bench guy who trash-talked everyone but got completely cut
6) Bob Uecker adlibbed almost everything he said, including "He one threw at his own kid at a father-son game."