Continuity mistake: When Eddie falls through the floorboards, you can see his arm looks perfectly fine and is unbroken in a few quick shots. After the movie cuts back to him a few moments later, suddenly his arm is badly broken. (01:20:30 - 01:23:25)
Continuity mistake: In the beginning of the movie when Georgie is running down the street chasing the paper boat his walkie-talkie is in his right raincoat pocket. After running a bit it switches to his left pocket.
Continuity mistake: When Beverly is signing Ben's year book she has the pen lid in her mouth. It disappears and then reappears.
Continuity mistake: The glass Bill uses to cut everyone's hands doesn't stay bloody for each character despite him never wiping it off. (02:05:05)
Continuity mistake: As Georgie is chanting at Bill "You'll float too! You'll float too!", Pennywise rises up from the water with crossed eyes whispering and wet hair down around his head and face. It cuts to a wide shot and now his eyes are normal, and his hair is parted and pushed back, while being less wet. (00:59:10)
Continuity mistake: At the quarry, there is an amusing scene where the boys all ogle Beverly. When she turns to look at them, they all pretend like they weren't staring at her. Pay attention to Richie during the next four shots. Richie looks to his right twice between the first two shots, and his legs go from crossed to uncrossed between the third and fourth shots.
Continuity mistake: Early in the film, Georgie draws a smiling face into the misted window in Bill's bedroom. When Georgie turns around to approach his brother Bill, the condensation and smiling face are gone. As the scene concludes, we see a close-up of the smiling face again, fading off the bedroom window. (00:01:00 - 00:03:00)
Continuity mistake: As they are coming through a tunnel in the sewer, Eddie's bandage is on his right cheek instead of the left for a brief moment. (02:00:03)
Continuity mistake: Whilst the losers are at Ben's house discussing the history of Derry, Beverly discovers a poster of "New Kids on the Block" (much to the embarrassment of Ben). She then closes the door to hide the poster. In subsequent scenes the door is closed and the poster is nowhere to be seen.
Continuity mistake: When Mike witnesses the burning hands through the butcher shop, he encounters the group of punk kids in the Pontiac Firebird. In the shot of the boys from the hood of the car, the chubbier kid who wore a trucker hat is driving. When they stop so Henry Bowers can yell at Mike, we see Henry is now in the driver's seat of the car instead of the other boy.
Continuity mistake: When they see the bullies messing with the black kid it shows all the kids' bikes - the kid with brown shorts is riding a rhino Turn and Burn which had a goose-neck braking system. That bike did not come out until 1998.
Continuity mistake: Ben is going down a small set of stairs towards one of the Easter Eggs. There is a support column there with the shadow coming towards Ben. In the next shot, the lighting is completely different now with light flooding the floor from behind Ben, casting the shadow of the column in the opposite direction. (00:27:05)
Continuity mistake: When Mike hides in an alleyway to avoid the bullies, he pulls a few packages out of the basket of his bike. He hears a noise at the door and looks at it with a package in his arm elevated a bit sticking up. There is a bang on the door as he stares at it, and then when it cuts to a view of the door from behind him the package is now lowered with his arm. (00:19:30)
Continuity mistake: When Gretta goes into the school restroom to confront Beverly, there is a backpack with a pink jacket on it sitting on the window. In the shot where Gretta is backing away from the stall before the trash is dumped into it, the pink jacket is in a different position with more of the backpack showing from under it. (00:12:05)