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.
Visible crew/equipment: In the scene were the adults go back into the sewer, and they have the feeling: "Something is coming", you can see the shadow of the cameraman, twice. One time, coming closer from a pipe, and another time, moving away. Look on the right wall/right on the floor.
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 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: 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: 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: In the last scene where that lady walks in and that guy is in the bathtub dead, it shows the guy with his hand in the bathtub but when it shows it again, the hand is on the wall with blood all over it.
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: 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.
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.