(at around 2h 5 mins) The glass Bill uses to cut everyone's hands doesn't stay bloody for each character despite him never wiping it off.
Bill's wristwatch switches from the right hand to the left hand through several consecutive shots.
(at around 48 mins) When Eddie leaves Ben's house to walk down Neibolt St, he is wearing completely different clothes.
(at around 12 mins) On the last day of school, some girls pour garbage and liquid muck on Beverly in the bathroom. She uses her book bag to partially shield herself. A short time later, when she talks to Ben outside, Beverly and her book bag are clean and dry.
(at around 16 mins) When Beverly Marsh first meets Ben Hanscom, she signs his empty signature book. When she does this, she removes the cap of her pen with her mouth, and holds the cap in her teeth while writing. The camera switches between Beverly and Ben several times while she is writing, sometimes the cap can be seen in her mouth, but other times it is missing.
Police cars are shown with red and blue lights. Police cars in Maine only display blue lights.
Ed has a broken arm with a cast on it. Ed accompanies his friends into the sewer were there is water everywhere. Even falling into the water which would destroy the arm cast but it seems undamaged in the end.
When the fight between Beverly and her father breaks out, she kicks him in the privates more than once and he goes down but miraculously gets up again. He wouldn't have been able to get up that quickly after being kicked like that.
(at around 1h 40 mins) When Bev is knocked out in IT's hide out, we see drops of blood fall. Three land on her left cheek, waking her up. Within seconds she is on her feet and the drops are no longer visible.
(at around 1h 25 mins) The hole in the floor that Eddie fell through is gone when Bill and Richie come out of the room with the mattress. This is most likely due to the fact that Pennywise created the hole in order to stop Eddie from getting to Bill and Richie. He probably returned the floor to its original state after Eddie fell through so that Bill and Richie won't know what happened to him, and increase their fear, due to them not knowing where Eddie is or what happened to him.
(at around 1h 24 mins) When Richie and Bill are upstairs in the Neibolt house trying to get to Eddie, they choose the door "Not Scary at All." When they open the door, a female voice asks, "Where's my shoe?" Bill reaches for and turns on a light. This shouldn't have been possible because the house would not have working electricity, however, given the fact the entire house can be manipulated in any way IT wants, the electricity is perfectly explainable, just as the body of a severed Betty Ripsom hanging in that closet is; it's entirely fabricated by Pennywise.
(at around 21 mins) When Stanley is in the synagogue reading out loud from a book in Hebrew, he is clearly holding the book upside down, revealing the actor cannot actually read Hebrew.
(at around 1h 6 mins) During the rock fight, the Losers hit several of the bullies with very large rocks, but they simply bounce off without the bullies even flinching.
(at around 1h 23 mins) When Ed breaks his arm, his forearm is suddenly longer than it's supposed to be.
(at around 37 mins) In the pharmacy, to the left, each shelf has price tags. On the right, none of the medical items have prices.
Throughout -the caps used to extend the clown's front teeth are quite apparent.
The woman hosting the TV show is actually telling the children to go play in the sewers.
(at around 56 mins) When Bill visits Georgie's room he picks up a green Lego turtle which later breaks on the floor. Green Lego brick other than plates for building structures on, or 1/3x1 plates did not exist in 1988/1989. Lego's reasoning behind this is they didn't want children building tanks from the bricks. The green 2x4 bricks shown were introduced in 1996 and the color lime (aka bright yellowish green) was introduced in 2001.
(at around 37 mins) In the first pharmacy scene, the beauty products on the shelves have modern day packaging. Most noticeable are the Dial and Ivory soaps and the Olay products.
(at around 37 mins) In one of the pharmacy scenes, a dome style security camera can be seen on the ceiling. This style of camera was not in use in 1989.
(at around 2 mins) The TV shown in the background in the Denbrough house is actually a Toshiba TV from the early 2000's. Also - the VCR (RCA) that is on top of the TV in the Kaspbrak house is from the late 90s/early 2000's.
(at around 37 mins) When Beverly meets the "Loser's Club" in the Pharmacy, Olay skin care products can be seen on the shelf behind her. In 1989 Olay products were known as Oil of Olay.
(at around 28 mins) Derry, Maine is in the USA, however a war memorial contains the line "for king and country", revealing the filming location in Canada.
(at around 1h 7 mins) After the "Rock Fight", when The Losers are walking, a train can be seen in the background. One of the freight cars has a logo of Canadian Tire, a Canadian hardware store. The film was shot in Ontario, Canada.
(at around 6 mins) The large, open storm drain shown in the film's opening scenes is not common in Maine and is more commonly seen in the Western USA.
During the rock fight scene, Richie is clearly seen getting hit by a rock in the head. Yet, in the reminder of the movie Richie is not shown to have any sign of a head injury and remains undamaged throughout the movie. He would have likely had a severe head injury, would have been sent to the hospital and possibly could have died.
Apparently the Derry police are way too busy to do anything about missing children.
From 0:51:54 -voices could not come up through the water in the sink's "P"-trap. And, if they were somehow able to, there should also be some bubbling noises.