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: At the start, George draws a face on the window before he asks Bill if he's sure he won't get in trouble. As he asks, he turns his body and looks at his brother, facing sideways with one arm stretched out over the window seal, his other arm partly lowered with just his hand on the ledge. It then cuts to a wide shot and now suddenly only George's head is turned with both of his arms up on the ledge and his body facing the window. (00:01:20)
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)
Suggested correction: There were several TVs similar to the one pictured in 1988. A quick Google search of "TV's available in 1988" will tell you this. Without knowing the manufacturer or model of the TV, you cannot state this as a movie mistake. If it was a LCD TV in 1988 then yes but, this one, no you can't assume this TV was not available.
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)
Continuity mistake: When Beverly is in the bathroom she puts out her cigarette on the cubicle wall using her right hand. The shot switches to overhead and she's now using her left. (00:12:10)
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)
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: Ben looks up at the librarian and asks if he can have the book now. His hand is beside the book in that shot, but it cuts to a wide angle from a distance and now his hand is on top of it. Also his mouth opens between shots. (00:25:05)
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)
Factual error: At the pharmacy on the shelf is Olay. It wasn't called that until 1999. It was Oil of Olay in 1989. (00:37:30)
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: 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: When the losers save Mike from Henry he is lying on the floor, with Vic on the left and Belch on the right. Mike flees - Henry stands up and says "you losers are trying too hard." As the camera comes back Belch and Vic have swapped sides. (01:06:00)
Continuity mistake: During the rock war, there's a slow motion shot of Belch screaming before Beverly then throws a rock at him. In the next shot of Belch, there is suddenly a cut on his nose before he ever gets hit. (01:06:50)
Factual error: During the rock fight, Richie calls Henry Bowers a "mullet wearing a**hole." The term "mullet" wasn't coined until 1994 by the Beastie Boys in the song "Mullet Head." (01:07:15)
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: When Miss K drops her keys and then tells Beverly to get back, she leans towards her threateningly. But her body is still turned to the side as her head faces her. It then cuts to a close up and now she is completely turned towards Beverly. (01:27:00)
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: 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: When they join hands during the blood oath, Bill's hand is nearest the camera, then the angle changes and Beverly's is. (02:05:45)
