Continuity mistake: In the race for the $100,000 purse (that Rosemont wins), just after Seabiscuit takes the lead, there's a closeup of Seabiscuit's head and Pollard's right leg - Pollard is wearing the brace he made later in the movie after his leg was shattered. Obviously, the shot was taken from those filmed for the Santa Anita race later in the film.
Factual error: Two men are trying to start a John Deere Model D tractor, which backfires, startling the horse. The wrong sound effect is used; it sounds like a V8 engine being turned over by an electric starter. The tractor's flywheel, on the left side, is shown stationary. Turning the flywheel by hand is the usual starting procedure for a JD tractor of '30s vintage. For over 40 years John Deere tractors used a two-cylinder engine, which make a distinctive popping exhaust sound. The movie is right on one thing, JD tractors could backfire.
Factual error: Pollard did not lose his sight in a fight, as the film suggests. He lost it when he was galloping a horse for exercise and a horse going in the opposite direction kicked up a clod of dirt that struck him at the base of skull and knocked out the cerebellum on his left lobe. (right side, left lobe).
Factual error: When Red Pollard is showing the doctor his homemade brace, his bare leg shows no sign of having been nearly severed or of undergoing multiple surgeries. There are no scars, atrophied muscles, or discoloration of any kind. In reality, Pollard's leg was permanently disfigured.
Deliberate mistake: In the final race of the film, Seabiscuit performs a flying lead change at the gallop while crossing the finish line, in the famous shot taken underneath his neck. Horses normally only switch their leads around turns while racing; it's something done to relieve pressure on the leading foot. From taking the shot over and over again, the horse playing Seabiscuit was probably tired out and a little tender and began to perform lead changes in other places in an attempt to be more comfortable.
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.)
Other mistake: When Marcela and Howard are viewing Seabiscuit for the first time (when he's weaving all over the place) it looks like it was done before as Seabiscuit follows a thin ribbon of disturbed ground that is weaving in and out. Its unlikely any other horse would be weaving like that.