During the parade, several of the people seen dancing (including the construction worker and the window washer) originally had nothing to do with the film. They were simply dancing to the music being played, and John Hughes found it so humorous that he told the camera operators to record it.
The idea of a sequel had gone around for years, with Ferris in college or on the job somewhere, but the idea was dropped. Matthew Broderick felt that the film didn't need a sequel, didn't need updating: the film is about a specific time and place that audiences would like to revisit.
John Hughes told Ben Stein, who had a degree in Economics, to present an actual Economics lecture in his scenes. Hence, nothing Stein says (aside from the roll call) is scripted.
To produce the desired drugged-out effect for his role as the drug addict in the police station, Charlie Sheen stayed awake for more than 48 hours before the scene was shot.
The shot of Ferris playing the clarinet was done on the spot. Someone spotted the instrument as part of the set. Matthew Broderick said he could play it, which of course he couldn't.
Louie Anderson: as a flower deliveryman in this film. He and Edie McClurg later lent their voice talent to Pains, Grains, and Allergy Shots (1995), whose title was based the John Hughes film Antes Só do que Mal Acompanhado (1987), in which McClurg also appeared.
John Hughes: Hughes can be seen in a tiny cameo in one of the early Chicago downtown montage sequences. He's climbing literally across traffic, from right to left of the screen, wearing a light blue jacket and big "'80s hairdo" (from the DVD director's commentary).
John Hughes: [school] Shermer High School is the same high school in Mulher Nota 1000 (1985), Gatinhas e Gatões (1984) and Clube dos Cinco (1985). The town name Shermer is also seen on a police badge.
John Hughes: [Shermer] When Jeanie is in the police station, you see a quick shot of the chest of a police officer including his badge which identifies him as a police officer in the town of Shermer.
John Hughes: [The Beatles] Ferris lip-syncs The Beatles' cover of "Twist and Shout". He quotes John Lennon's song "God" ("I don't believe in Beatles, I just believe in me.") Cameron's Detroit Red Wings jersey references Paul McCartney and Wings, as his Epiphone Texan acoustic guitar (which he played on The Beatles' "Yesterday") sported a Detroit Red Wings sticker from the mid-1970s onward. During filming, Hughes "listened to The White Album every single day for fifty-six days".