Revealing mistake: In the scene where Stripes is looking in the water at his reflection, his body and the reflection don't quite match up. You can tell by his movements that his reflection is computer generated. (This is verified by the audio commentary.)
Racing Stripes (2005)
1 revealing mistake - chronological order
Directed by: Frederik Du Chau
Starring: Bruce Greenwood, David Spade, Hayden Panettiere, Frankie Muniz, Snoop Dogg, Caspar Poyck, Gary Bullock
Deliberate mistake: As the horse named "Sir Trenton", who is supposed to be a stallion, is galloping away you can see that "he" is a female.
Scuzz: Try starting out life as a maggot. Nobody wants to pet a little maggot.