Acting for Two: Barrie Ingham voices both Basil and the unlucky Bartholomew.
Awesome, Dear Boy: Price on his role as Ratigan. He had stated that Ratigan was his favorite film role.
Dueling Movies: With An American Tail, another animated movie about mice coming out the same year (though it didn't come out until a few months afterward). The Great Mouse Detective lost in the end, but that didn't mean it wasn't successful; in fact, it convinced Disney that its animation unit still had a future.
The success of An American Tail probably helped convince Disney as well.
Executive Meddling: Largely averted, as Jeffrey Katzenberg had little to no faith this movie would be any good. Most of his attention was spent on meddling with The Black Cauldron, which became a failure. Katzenberg did, however, demand the title of this film be changed from "Basil of Baker Street" to simply "The Great Mouse Detective", a move that inspired an infamous fake memo that became a category on Jeopardy. Two of the four directors have since said they've always hated the new title.
However those directors may have had some solace in the title being changed to "Basil the Great Mouse Detective" is some countries, including Australia.
Though it's rather hard to tell, the lady mouse who comes to see Basil at the end of the film is none other than Nancy herself!
Hilarious in Hindsight: The box that Olivia is sitting on when Dawson finds her reads "Dr. Gaston's Liver Pills." 5 years later, we would meet Dr. Gaston, and he was evidently health conscious, based on eating 4 dozen eggs every day.
I Am Not Shazam: The lead dancer at the bar is listed in the credits as "Miss Kitty Mouse," but, Fanon aside, this doesn't actually appear to be her name. Rather, it's a description of her. She isn't "Miss Kitty Mouse," but "the Miss Kitty mouse."