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 Beverly kisses Bill, in one angle his ear is between her fingers, from another her entire hand is on his cheek. The blood smear she leaves behind also changes. (02:08:20)
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: 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)
Continuity mistake: When Beverly is signing Ben's year book she has the pen lid in her mouth. It disappears and then reappears.
Trivia: During the scene while Ben is reading in the library, there is a shot where if you look in the background behind Ben, you can see the librarian giving him a very devious look. This strongly implies that she is being controlled by Pennywise. The camera isn't focused on the background, which means it's very easy to miss if you're not looking for it, but if you're looking for it, you'll surely see it.
Factual error: When Bill meets Beverly for the first time and the shot is showing her while walking towards the other guys, some modern SUV are visible in the background.
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: 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: 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: 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: 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)
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)
