Continuity mistake: In the part where the 7 kids are looking at Mike's photo book, Pennywise turns the page to the picture that comes to life. When the page is first flipped, you can see that on that page, there are a few pictures on that page. But, when Stan throws the book on the ground, that picture is the only one on that page.
Trivia: When the adults walk into the library, you'll see some of Bill's books on display (Bill is described as the Stephen King character), and all of Bill's books (Whether it's the name, topic, or cover picture) are based in some way on Stephen King's books. For instance - The Glowing/The Shining.
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.
Bill: He thr-thr-thr-thrusts his fi-fi-fists ag-ag-against the pos-posts and...
Ben: He thrusts his fists against the posts and still insists he sees the ghosts. That's all it says over and over.
Bill: Th-th-thats w-w-what m-m-my m-m-mom g-gave me t-to h-help with my st-st-stutter.
Richie: I hate to tell you buddy, it ain't working.