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: 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)