Factual error: During the opening scene in 1988, a silver TV from the late 90's/early 00's is in the living room/dining room area. (00:02:10)
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)
Factual error: Eddie specifically tells the gang not to take his mom's "Delicious Deals" snacks because his mom loves them. But Delicious Deals are made by Mrs. Freshley, a company that was not founded until 1994. The boxes used in the movie even include the modern design that can be found today in most Dollar Tree stores. (00:23:00)
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.
Factual error: When Stanley is at synagogue practicing for his Bar Mitzvah, and the Rabbi tells him to close the book, he does so - and a clear shot of the book cover from Stanley's left shows that the book (in Hebrew) is upside down. Hebrew books read right to left - the cover should be on Stanley's right, right side up.
Factual error: Police cars are shown with red and blue lights. Police cars in Maine only display blue lights.
Factual error: Bill is holding a Lego turtle and drops it on the floor, then you see a closeup on the parts. Green Lego bricks were not commercially available before 1996 and lime green Lego was not available before 2003. (00:56:20)
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)