Revealing mistake: In the library scene when Richie sees Pennywise the clown, all the balloons filled with blood are popping on people who aren't supposed to be able to see them, but one lands on a lady and you can see her jump as the balloon pops.
In Derry, Maine, there is a monster that takes the shape of whatever you're afraid of, and kills/eats kids. It soon targets seven friends - Bill, Ben, Eddie, Beverly, Mike, Richie, and Stan - after killing Bill's little brother, Georgie. Eventually the kids kill IT, but thirty years later (twenty-seven in the book by Stephen King), IT comes back to Derry. Mike, the only one of the seven who stayed in Derry, calls the six others to come back and kill IT (they took an oath to after they first killed IT), and all come but Stan (who kills himself) to finish off IT once and for all.
More mistakes in It
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.
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.
Question: What is the deal with everyone saying "beep beep" to Ritchie every time he tells a joke?
Answer: In the book it is explained that they are telling Richie to shut up.
Chosen answer: It's their way of telling Richie to be quiet. To get him to stop talking.