(at around 32 mins) Tom flicks Bev's hair behind her shoulders but she quickly puts it back in front when Tom leaves the room. But when her and Tom are walking to meet the Japanese men, her hair is back behind her shoulders again.
(at around 58 mins) In the cafeteria, Henry Bowers smears mashed potatoes on Richie's glasses. Later, Richie's glasses are clean.
(at around 12 mins) After Georgie is killed, we see a brief funeral sequence. Attending are Beverly and Ritchie, who have not yet met Bill. This can be explained away because they are all in the same class at school, although it doesn't explain why they all share the same classroom, and still don't know each other.
(at around 23 mins) When Henry's gang is chasing Young Ben, a long shot establishes there is water running out of the drain pipe in the side of the hill, but when Young Ben hides in it, there is no running water.
Young Beverly's hair changes color several times.
(at around 52 mins) When Eddie is traveling from NY to Derry, he leaves from Penn Station. In the shot of him catching the train, you see that the train station is clearly outdoors. Penn Station is completely underground and therefore no outside scenery would be visible.
(at around 40 mins) The song "It's Alright" by The Impressions wasn't released until 1963; the early part of the story is set in 1960. However, the song is non-diegetic, which means that it is not heard by the characters.
(at around 10 mins) When Georgie is playing with his newspaper boat in the rain, shadows of many objects can be seen and rays of sun are visible throughout the scene. However, it is quite possible, and even common in many areas of the world, for it to be both raining and have part of the sky not covered in clouds, resulting in sunlight showing while it is raining.
(at around 1h 16 mins) Explosive charges clearly visible inside the glass bottles when Bev tries out the slingshot.
(at around 1h 6 mins) After Young Ben kicks Young Henry, then falls down a bank into the barrens, it's easy to tell that it's a stuntman falling because a glimpse of their face is visible. The stunt man's hair is much darker than Young Ben's.
(at around 42 mins) When young Beverly is greeted by the voices from out of her bathroom sink. You can see the reflection of the red balloon on the shiny plughole, that will blow out in a few minutes.
(at around 50 mins) When Eddie is in the shower, as he puts his towel back on, there is a flash of the modesty pants he was wearing for the scene.
(at around 10 mins) When Georgie is playing with his newspaper boat, you can see the string that is pulling it along the gutter.
(at around 41 mins) When Ben, Bev and Mike are parting ways after building the dam, Ben heads down an alley with what is clearly a cube van from the 1980s.
(at around 48 mins) The flashbacks to 1960 seem a little confused as to what specific time of year that Round One with Pennywise occurred. When the first six members of the Losers' Club get back from seeing I Was a Teenage Werewolf (1957) (which, by the way, actually came out in June of 1957), Eddie wishes that "this summer would never end." Yet, the LC--indeed, all the kids in Derry--are still attending middle school.
(at around 30 mins) When Pennywise turns into a skeleton and menaces the young Ben, Pennywise's words don't match the moving of the skeleton's mouth.
(at around 1h 28 mins) When Stan is reciting the Boy Scout motto in the sewers, his lips and the words clearly don't match up.
(at around 6 mins) When Mike calls Bill's, it appears to be late at night in Derry. Bill is in England which is 5 hour ahead difference. Yet Bill is just getting ready to have dinner as if it was still early.
(at around 49 mins) During Eddie's flashback scene, when he and the other Losers arrive at his house he tells them, "I wish this summer would never end; it's the best summer of my whole life," just before his mother comes out. His mother then tells him she doesn't want him to take gym in school that week. If it was summer vacation he would not have had school that week, yet the kids are shown in school repeatedly.
(at around 27 mins) Ben describes Elmer Fudd's speech impediment as a lisp. Elmer mispronounces the letter R, a condition known as rhotacism. People with a lisp mispronounce the letter S.
(at around 1h 17 mins) The first time Stan recites the entire Boy Scout Oath, he misses the line "To obey the Scout Law". This line goes between the lines "To do my duty to God and my country" and "To help other people at all times".