In the Walrus and the Carpenter sequence, the R in the word "March" on the mother oyster's calendar flashes. This alludes to the old adage about only eating oysters in a month with an R in its name. That is because those months without an R (May, June, July, August) are the summer months in England, when oysters would not keep due to the heat, in the days before refrigeration.
This movie has more songs and characters than any other Disney animated film.
Lewis Carroll wrote the riddle "Why is a raven like a writing desk?" as nonsense - it has no answer. This has not stopped people, despite being repeatedly told that there is not, nor should there be, any answer, from trying to contrive one. Among the suggestions are, "Because Edgar Allan Poe wrote on both" and "Because the notes for which they are noted are not noted for being musical notes" (the second of which is very similar to a solution that Carroll himself wearily suggested when he grew tired of people asking him about it).
The movie took five years to complete but was in development for over ten years before it entered active production.