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.
The movie took five years to complete but was in development for over ten years before it entered active production.
While filming the live-action reference scenes for the Mad Tea Party, Ed Wynn ad-libbed the speech where the Mad Hatter tries to "fix" the White Rabbit's watch. ("Muthtard? Don't leth be thilly!") Walt Disney was watching the filming, and told the animators, "Hey, that stuff's pretty funny. Why don't you use that speech in the movie?" The animators objected. "We can't use that. There are too many background noises on the film." Disney smiled, and told them, "That's *your* problem," then walked out of the room. Eventually, with much labor, the Disney sound technicians managed to re-record Wynn's dialogue and erase all the background noises, so that Wynn's ad-libs were used in the final animated film. (The original live-action footage still exists, and has been featured as 'Bonus Material' on Alice au pays des merveilles (1951) DVDs.)
The Mad Hatter is apparently based on the phrase "mad as a hatter" which has a historical basis. In the hat industry of the 19th century there was extensive use of mercury. Liquid mercury is one of the causes of mercury poisoning, which (among other things) causes brain damage. Among the symptoms are slurred speech, memory loss, tremors, and behavior that can be seen as insane.