(at around 20 mins) When adult Bev leaves her father's house and is standing in the middle of the street, the truck coming at her is honking its horn but the driver has both hands on the steering wheel.
(at around 17 mins) When adult Beverly is seen at the door of Mrs. Kersh's house, the doorbell tag says MARSH, then "Marsh" with lower case "arsh" as the camera cuts to a close-up of the doorbell, then flips back to "MARSH" in all caps in the following wide-shot. The close-up insert shot featuring the lower case letters was a later pick-up, to make sure the viewer noted the name Bev was seeing, to make it more dramatic when she looks back and finally sees the name Kersh.
(at around 57 mins) When Ben grabs Bev's shawl, he picks up a white one with blue flowers on it, but when Bev shows up, and as he looks around the room before leaving, it is a purple and pink striped shawl.
(at around 56 mins) In Part 1 (1990), when the kids go down into the sewers, young Bill tells the others, "All right, now stay together, no stragglers." But when adult Bill is having a flashback of that event, young Bill says, "All right, now listen up. Stay together, no stragglers."
(at around 42 mins) When "He thrusts his fists against the posts and still insists he sees the ghosts." is being ghost-typed in the library, it is aligned going down the left side of the paper in short segments, but when Ben holds the paper and starts reading it, the shadow through the back of the paper shows the phrase to be typed in lines across the top of the page.
(at around 6 mins) When Richie sees his epitaph on a cinema billboard, it says he was born in 1950. The events of his childhood take place in 1960, making him ten years old, not twelve years old like he and the rest of the gang are supposed to be.
Actress Helena Yea, who plays Rose, has her name spelled wrong in the credits as "Helena Yee."
(at around 1h 22 mins) The spider in the cave has only six legs. Thats wrong. Insects have six legs - spiders have always eight legs.
(at around 18 mins) When adult Beverly visits her childhood home and goes to the bathroom, she turns off the water with a couple inches of water in the sink. A moment later, when Mrs. Kersh calls for her, the sink is about two thirds full. However, this is intentional. Everything that happens to Beverly inside the house is actually an illusion created by Pennywise.
(at around 7 mins) The people in the library jump when Pennywise's balloons pop in front of them, even though they're not supposed to see or hear them.
When Eddie arrives in Derry and gets out of the cab, a Canadian flag can be seen in the background.
(at around 13 mins) When Ben enters the cab the second time we see a brief scene of Pennywise waving and laughing at Ben ominously. When we cut, and in the background we can see Pennywise still waving, even though he should have disappeared because Ben is the only one who can see him.
(at around 9 mins) Stuntman's face briefly visible when Mike rides Silver across the see-saw.
(at around 1h 19 mins) When adult Bill returns Georgie's boat to the sewer so that the boat will lead them to It, you can see the string attached to the front of the boat, causing a wake.
(at around 9 mins) When Bill and Mike are about to put bicycle playing cards in the bike's wheels and Bill drops the cards, there are two Ace of Spades cards face up. A deck would only have one Ace of Spades. The Pennywise backs are an otherwise obvious change.
(at around 42 mins) When the adults are in the library and the books are flying off the shelves, look below the second shelf for a brief flash of a metal object running the length of the shelf. It appears to be the device used to knock the books off the shelves.
(at around 20 mins) After Bev has been chased out of her father's house 30 years later, If you look to the far side of the screen you can faintly see the thin metal stick which they tied to the balloon so that one of the crew could make it bounce from a distance.
(at around 1h 15 mins) When the Adults are in the sewer and Georgie's paper boat is coming down the tunnel towards them, the lights are casting the camera's shadow against the wall, so you see the camera following the boat.
(at around 13 mins) When the now adult Eddie gets out of his car upon returning to Derry, right before he spots the pharmacy from his childhood and says, "aw hell, time does go by!", there's a background shot of a tall building behind him. Atop it is a Canadian flag with its red-and-white colors, as well as the phrase "Columbia Days" painted on the pharmacy window (the film was recorded in the province of British Columbia), however the story is set in Maine, the United States.
(at around 8 mins) When Adult Bill is fixing the tire on his bicycle, among the scatter of tools on the ground there is no evidence of the air pump he would have required to re-inflate the flat tire on the bike.
(at around 1 min) As Pennywise gloats over the seven graves dug in the Barrens, we see a cross over the grave of Stan Uris. Uris is Jewish, and would not have a cross on his grave.
(at around 37 mins) When Stan runs in to Bill after his encounter with the Mummy. Older Bill, who's narrating the event, states that he never saw anything because he was pedaling too fast to turn around and look. However, when they are riding away, you can clearly see that Bill is riding his bike uphill without pedaling even once.
(at around 47 mins) Eddie doesn't always use his inhaler properly. When at the hotel, you can see him putting it to his mouth, but slightly to the side so the spray doesn't go into his mouth.
(at around 48 mins) In Part 1 (1990), when Mike is in front of the class, he says that the disaster in 1900 had to do with the Derry Standpipe. When he is older and explaining to the group a disaster every 30 years, he says that the 1900 disaster was the massacre at Drake's Creek, not at the Derry Standpipe.