From the
Wizard of Oz IMDB page:
When the wardrobe department was looking for a coat for Frank Morgan (Professor Marvel / The Wizard), they decided they wanted one that looked like it had once been elegant but had since "gone to seed". They visited a second-hand store and purchased an entire rack of coats, from which Morgan, the head of the wardrobe department, and director Victor Fleming chose one they felt gave off the perfect appearance of "shabby gentility". One day, while he was on set in the coat, Morgan idly turned out one of the pockets and discovered a label indicating that the coat had been made for L. Frank Baum. Mary Mayer, a unit publicist for the film, contacted the tailor and Baum's widow, who both verified that the coat had at one time been owned by the author of the original "Wizard of Oz" books. After the filming was completed, the coat was presented to Mrs. Baum.
This is by far my favorite movie-related "coincidence." It's awesome.