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.
Continuity mistake: When the 7 kids throw stones at Henry Bowers he gets hit, falls down and starts to bleed on his forehead. When he later gets up and walks away there is no bleeding on his forehead.
Continuity mistake: When they are in the library after they found out Stan committed suicide, it shows Beverly stand up but when it cuts back to her she is sitting down and stands up.
Continuity mistake: When the clown is causing the library to be the target of a windy rainstorm, the books are being pulled off the shelves row by row. They cut back to the shelf where the books have already been pulled off, and yet there are still many on there!
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: When Ben is running from the trio, he goes into the Barrens and spots a drainpipe to hide in. The pipe has a smooth-surface and has water running from it. When it shows Ben hiding in it, the pipe is rifled and there's no water coming out of it. It's not even wet on the inside.
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.
Continuity mistake: Young Bill has a dimple in his chin that old Bill doesn't have.
Factual error: In 1960 young Ben, Beverly, and Bill are all taking leave of each other. Ben starts to walk away as Bill rides his bike away; Beverly muses "My heart burns there too" while looking longingly after Bill. A white 1980s-era box truck is parked down the street in the direction that Ben is heading.
Revealing mistake: When they're having a rock war, Henry throws a rock at Beverly. When the rock hits her, it bounces right off her, making it obvious that it was a rubber or bouncy ball, and not a rock.
Revealing mistake: When Henry is about to carve his name into Ben, he kicks Henry and falls back through the fence. As he is rolling his hair is darker than usual, making it obvious that it was a stuntman.