Trivia: At the library IT writes something on the type-writer, a verse which Bill tells the others his mother gave him as a kid to help with his stuttering. The verse goes "He thrusts his fists against the posts and still insists he sees the ghosts." In the book, young Bill used this verse to defeat IT in the Chüd ritual.
Suggested correction: In the book they don't find out about the ritual until they are adults. Adult Mike is the one who told them about it after years of research.
Other mistake: When Georgie is following the paper boat there is construction equipment on the road that he goes around, there is something burning there but it's pouring rain.
Suggested correction: Those are oil lamps, they can keep burning in extreme conditions, including rain.
Factual error: The kids are leaving a movie theater that's playing I Was a Teenage Werewolf. This film was released in June of 1957. So one, they are in 1960 and this film would not still be playing. And two, they were shown still attending school, but they would not be attending school in June during summer break.
Suggested correction: Before the rise of home video, movies had much longer theatrical lives. It was not uncommon for a film like this to be shown as a Saturday matinee years after its initial release. I personally remember a local theater showing movies from the 60s well into the 70s.
