Although Walt Disney never knew it, he himself was character designer Bill Peet's model for Merlin. Peet saw them both as argumentative, cantankerous, but playful and very intelligent. Peet also gave Merlin Walt's nose. This was the second instance in which Walt unknowingly served as model for a wizard, the first being the wizard Yensid from the Sorcerer's Apprentice in Fantasia (1940). This explains why the character was given the name Yensid. This read backwards is Disney.
The first Disney animated feature with songs by Richard M. Sherman and Robert B. Sherman. This pair created some of the studio's best songs going into the next decade -- probably the most famous of which are the songs from Mary Poppins.
The last film in which Bill Peet served as a writer. He later created a version of The Jungle Book (1967) but Walt Disney threw Peet's story out when their relationship fell apart and Peet left the studio.
This was the first Disney animated feature made under a single director. Previous features were directed either by three or four directors, or by a team of sequence directors under a supervising director. The man hired for the job was veteran animator Wolfgang Reitherman (one of the fabled Nine Old Men), who would direct all of the Disney features up until the 1980's.
In the Squirrel scene, Merlin tells Wart that when a "girl squirrel chooses her man, it is for life!". In reality, however, the male and the female squirrel spend only a couple of nights together in the female's nest during a mating season (usually twice a year, one between December-February and the other between June-August), after which the male leaves the nest forgetting all about his family. It is the female then who is responsible for raising the baby squirrels.
Two songs written for the film but scrapped before production began were "The Blue Oak Tree" and "The Magic Key". "The Blue Oak Tree" was sung by Sir Ector and Sir Pelinore when they celebrated Kay's knighthood and "The Magic Key" was to be Merlin's lecture to Arthur about the value of an education. It was replaced with the more amusing "Higitus Figitus".
Many elements of Sleeping Beauty (1959) have been recycled into films such as The Sword in the Stone (1963), which reuses opening credit backgrounds and various animation sequences. The two most noticeable are the owl from the forest scene, who would inspire Archemedes, and Malificent in dragon form, which led to Madam Mim in dragon form.
In the Squirrel scene, Merlin transforms himself into a gray squirrel, while Wart turns into a red. In reality, gray squirrels where imported into England (from their native north America) no sooner than 1870s, after which they gradually dominated all of the forests and made the red squirrel almost extinct on the British Islands. The story, however, takes place apparently during the Middle Ages, long before that time. It might be a deliberate feature in the movie, as Merlin is capable of seeing deep in the future.
The trivia items below may give away important plot points.
Arthur was voiced by three different boys - Rickie Sorensen, Richard Reitherman and Robert Reitherman. The changes in voice are very noticeable in the film because of the way Arthur's voice keeps going from broken to unbroken, sometimes in the same scene. One of the easiest noticed is in the last scene in the throne room when Arthur asks in his "changed voice", "Oh, Archimedes, I wish Merlin was here!" Then, the camera cuts farther back and Arthur shouts in his "unchanged voice," "Merlin! Merlin!"
The climactic battle between Merlin and Mad Madam Mim is often cited by animation experts as some of the best character animation to that date. The characters go through numerous physical transformations during battle, yet retain their identifying features; Merlin's guises are blue and include his glasses and facial hair, while Mim's are pink and purple and have her messy hair.