Rosie O'Donnell has stated that the character of Roberta was supposed to be a lesbian, but the film was later re-edited and she was made straight. The line, 'Roberta lives in sin with her boyfriend." was looped in at the last minute.
In the scene where Chrissy punches Roberta in the face for pretending to drown, 'Christina Ricci' forgot to turn her head and ended up getting punched in the face full force. The production was shut down few days due to Christina being badly bruised.
When the adult Chrissy is scanning the front cover of "People" with Teeny on the cover, the smaller headline at the top reads, "Linda Richmond tells all in new autobiography!" with a picture of Mike Myers dressed as his famous Saturday Night Live (1975) character.
In the movie they are singing the Tony Orlando hit song Knock 3 Times while on their way to do research at the library. The song was actually not released until November 1970 and therefore would not have been possible to listen to in the summer of 1970, when the story takes place.
The movie (aka "The Gaslight Addition") was actually written about a town in Indiana named Winchester, and its gaslight addition. The author of the story I. Marlene King grew up there, as did the director Robert Wise, in vastly different years. Winchester declined to have their name associated with the movie, so the name became Shelby, but later Winchester began to preserve areas mentioned in the movie.
At one point, one of the girls sarcastically asks, "What are you, a pod person?" This is a reference to the sci-fi horror film Invasion of the Body Snatchers (1956), which would have been quite popular among twelve-year-old girls and other young people during the period portrayed in the film.