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)
Factual error: When passing under the railway bridge not only are the cars modern but there is also a Canadian Tire container on it which is a Canada only store, and doesn't ship by rail in the USA.
Factual error: While Eddie and Bill are in the prescription section of the store, there is a security camera on the ceiling that wasn't released in the 80's.
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.
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: 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)
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)
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)
Factual error: Ben's headphones look like 1995 plastic models and not 1989 style.