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.
Continuity mistake: The blood splatter in the restroom scene splatters Beverly in the face with a noticeable amount of blood. In the next scene where Beverly's dad is in the restroom, the quantity of blood has decreased.
Continuity mistake: When grown up Beverly is throwing stuff at her boyfriend, she picks up a bottle of rocks or something like that and throws it at her boyfriend but when it gets to him, it has magically changed into a shampoo bottle. (00:35:15)
Continuity mistake: When the bullies grab Richie in the cafeteria and pull him away, his plate has peas in the front and mashed potatoes on the right side. Then when Henry takes some potatoes and smears them on Richie's glasses, the potatoes are at the front of his plate, and the peas are at the back.