Revealing mistake: After William Macy interviews Jeff Bridges and when the narrator is describing Seabiscuit's fans (the scenes are about an hour apart), you see shots of people in trees. They are the same shot.
Revealing mistake: In the beginning scene, where the horse tamer is chasing the "mustangs", you can occasionally see the glint of a horseshoe on various "wild horses". (00:03:40)
Revealing mistake: In one scene we get a view of people standing beside a vintage automobile. A license plate is mounted on the vehicle's rear fender and we get a clear view of the back of the license plate. The plate appears to be made out of aluminum and pressed into the plate is a relief image of an antique automobile. What we are seeing is a modern license plate issued by some state for a vintage automobiles. (In Iowa a car must be at least 25 years old to qualify for such a special plate.)
Revealing mistake: After the race where Seabiscuit beats War Admiral, we see the train on the return trip home. If you look closely, the engine number on the locomotive is reversed, indicating that the film was flipped.
Answer: Red asked for $10 because he needed to see a dentist, and Howard gave him $20.