The narrator of the Mr. Toad segment, Basil Rathbone, mentions Sherlock Holmes as one of many possible nominees for the most fabulous character in English literature. This is taken a step further by Ratty, who wears a deerstalker cap and a coat very similar to Holmes' trademark garb, and Moley could pass very easily in appearance as an animal Watson.
If WWII hadn't swallowed up so much of Disney's workforce they would've developed these shorts into two full-length features rather than truncate them into their current form. The Mr Toad section of the movie was originally going to be much different - for one thing, the jailer's daughter was the one who gave Toad his female disguise instead of Cyril. For more information, see article here.
After his role as emcee of sorts in Fun and Fancy Free, it was proposed that Jiminy Cricket again play that same role for this film, but it was dropped.
When it was decided that the adaptation of Willows would not be a feature-length film but rather part of a package film, discussions were held on what to pair it with:
The first concept was to pair it with Mickey and the Beanstalk and The Gremlins in a film called Three Fabulous Characters, which became Two Fabulous Characters after Disney abandoned their attempt at adapting Gremlins in any movie form following the end of WWII. Willows itself was then removed from the project and replaced with an adaptation of Bongo, turning Two Fabulous Characters into Fun and Fancy Free.
The second concept was to pair Willows with re-tellings of the legends of cowboy Pecos Bill and engineer "Casey" Jones in a film titled All in Fun, before the Pecos Bill short was removed from the project and made the final segment of Melody Time. The "Casey" Jones short followed suit shortly after and was released on its own as a seven-minute short in 1950.
When the narrator mentions “Christian charity” in regards to Mole and Rat, it not only foreshadows their role in Mickey's Christmas Carol, it might also be the only outright religious reference in a Disney Animated Classic. (Apart from all those mentions of Christmas... and of course The Hunchback of Notre Dame.)
Likewise, among the American characters Bing Crosby mentions during Ichabod Crane's introduction were Paul Bunyan, Pecos Bill, Johnny Appleseed, Black Bart, Davy Crockett and Daniel Boone. Bunyan and Crockett were later adapted by Disney for later projects (Bunyan became an animated short, while Davy Crockett's life was adapted into a television series and several full-length movies, reintroducing him to the American public in the process.) On top of that, both Johnny Appleseed and Pecos Bill had already been the subjects of their own segments in Melody Time, the film released immediately before this.
Thurl Ravenscroft was supposed to sing "The Headless Horseman Song". You can find his version here.