Butters says that he watched all 132 episodes of The Simpsons (1989). At the time this episode aired, there were 291 episodes of The Simpsons (1989) aired.
When Butters tells General Disarray about his idea for the monorail, Dougie says the Simpsons did it in Episode 204, but the actual episode was the 12th of Season 4, episode number 71.
Contrary to what is said here, no character on the Simpsons has ever ran away and joined the circus. The closest thing was "The Simpsons" (1989) {Bart Carny (#9.12)} in which Bart and Homer have to work off debts at a carnival.