The name and life dates of Frank's muse Dorothy are portrayed inaccurately (see trivia).
Kenneth Gage Baum, the fourth son of L. Frank Baum and Maud Gage Baum, was omitted from the film, although he was present for much of the time period that the film shows.
The logo of the Chicago publisher (who finally accepts Frank's Emerald City book) reads "George Hill." In fact, the name was "George M. Hill."
During the theater fire, someone calls Baum "a method actor". Method acting wasn't invented until about twenty years later in the early twentieth century.
Around the time the Father Goose poetry book comes out, Frank says he is 40. In fact, L. Frank Baum was closer to 43.
Baum winces at Denslow's cigar smoke. L. Frank Baum smoked cigars himself, and resorted to chewing unlit cigars in public on allegations of doctor's orders.