WARNING: Contains spoilers from the film AND novel versions of It.
1. When Ben is reading through a book in the library, there's a terrifying elderly woman lurking in the background with a sinister grin on her face.
2. Before Patrick Hockstetter encounters Pennywise in the sewer, he sees an "I heart Derry" balloon. This is a reference to a character in the book named Adrian Mellon who wears an “I heart Derry” hat. Adrian is eventually thrown off a bridge by bullies and then dragged into the sewer by Pennywise.
3. In the novel, "the intersection of Witcham and Jackson" is mentioned when Georgie's boat is bobbing down the flooded street, and we see that in the film.
4. When Richie gets locked in a creepy room full of clown dolls, one of them looks like the 1990 version of Pennywise played by Tim Curry.
5. Richie is seen wearing a Freese's shirt. Freese's was a popular department store in Maine for many years.
6. In the book and 1990 miniseries, Richie has a fear of werewolves after watching I Was a Teenage Werewolf. The 2017 film doesn't mention this, but the group encounters Pennywise in the Neibolt house, and we can see a werewolf hand bursting through his clown glove.
7. A copy of The Frog Prince is seen in Beverly's room. This foreshadows the kiss she receives from Ben that wakes her from her trance.
8. The novel described a group of outlaws called "The Bradley Gang," who are killed by the people of Derry in 1929. During the film, when Mike has visions of burning hands creeping through the crack of a door, a mural of The Bradley Gang's shootout can be seen.
9. Eddie is seen wearing a shirt that depicts an angry-faced vehicle. This is a reference to another Stephen King novel turned film, about a supernatural car that kills, titled Christine.
10. In the novel, Richie encounters a giant Paul Bunyan statue that comes to life and tries to kill him. That doesn't happen in the film, but a Paul Bunyan statue can be spotted.
11. In the novel, there's a mysterious being called The Turtle, who is Pennywise's enemy. In the film we see Bill pick up and drop a Lego turtle in Georgie's room. There's also a scene in which the kids are swimming in the lake, and a turtle is spotted.
12. Bill is seen in a shirt that says "Tracker Bros." In the novel, there's a shipping company in Derry called Tracker Brothers.
13. The label on the beer bottle Richie breaks outside the Neibolt house says "Bob Gray's." Bob Gray is a name Pennywise goes by.
14. When the kids part ways at the end of the film, the order they leave is believed to be symbolic. Stan leaves first, followed by Eddie. In the novel, this is the order in which they die as adults — Stan, then Eddie.
CORRECTION
An earlier version of this post misstated Eddie's job as an adult.