When Disney was trying to develop a Mickey Mouse feature film in the early 1940s, one of the ideas considered was "The Three Musketeers," but it was abandoned because the original novel requires four main characters (the titular musketeers and D'Artagnan), and the filmmakers found it difficult to come up with a good combination of established characters to fill the parts. Sixty-plus years later, the problem was solved by having Mickey, Donald and Goofy not play the original Three Musketeers and thus tell a story that parallels the original, but is not strictly an adaptation of it. The original Musketeers, incidentally, are the ones who saved the gang in the beginning of the film, and their autographs are in the hat they give Mickey.
This was the first feature length movie starring Mickey Mouse, Donald Duck, and Goofy together.
Goofy is the only main Disney character (out of Mickey and Donald) to not appear in his classic outfit.
When Pete has captured Mickey, he recites some of the lyrics to 'The Mickey Mouse Club March' ("Hey there! Hi there! Ho there! You're as welcome as can be!").
One of Peg-Leg Pete's schemes involves having one of his henchmen disguise himself as the princess Minnie in order for "her" to publicly declare him her successor. This mirrors Ratigan's plot of creating a robot in the image of the Queen in order for it to do the same in The Great Mouse Detective (1986). Pete even puts on a robe identical to that which Ratigan wears in that very scene.
Mickey's hat is signed by Athos, Porthos, Aramis and D'Artagnan, the original Musketeers.
Interestingly the "opera " songs and music in the film are not operatic music at all bi but from operettas. A light form of opera and a totally separate genre. The only music the is truly from an opera is clarabelllas song whis is from the opera "carmen" by bizet.
When Pete puts Mickey in the dungeon, he says he calls the name of the Opera "I Just Can't Wait To Be King," which is the name of the song in The Lion King (1994) where Jim Cummings voiced Ed and did Scar's singing voice for the song "Be Prepared."
The songs in the opera are all from 'The Pirates of Penzance' but not in the correct order and not presented in the same way as in the play ("Poor Wandering One," for example, is a solo but is performed by a trio in this film).
Pete's horse drawn carriage has the license plate A-113, like many vehicles in Pixar releases