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.
First 30 min of the movie: When Beverley was signing on Ben's autograph, she pulled the cap of the pen using her teeth. The next 4 wide shots showed Beverly's teeth with cap, no cap, with cap, and no cap, all in the same scene.
The town of Derry is set in the state of Maine, however, when a war memorial is shown in the town, it commemorates men who fought and died in the great war "for king and country" revealing the actual filming location as being in Canada.
When Beverly is floating in the sewers, the camera keeps changing back and forth to her, and on several occasions, she goes from wearing red leggings to no leggings underneath her dress.
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.
The glass Bill uses to cut everyone's hands doesn't stay bloody for each character despite him never wiping it off.
Police cars are shown with red and blue lights. Police cars in Maine only display blue lights.
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.
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.
One of the televisions at the start of the film in one of the houses is a silver Sony CRT that was typical of the early 2000s rather than the late 1980s.
After the fight at the Barrens, the bully's head wound is still there 2 months later.
Some contemporary cars are parked in the background in some shots, but these are very brief.
When the boys are at the pharmacy buying things for Ben, behind them there are Nivea products that come out after 2010. Including a skin conditioner to use in the shower. And some Garnier creams, which was founded in 1904.
When Bill and Beverly kiss, the location of the blood on Bill's face changes between cuts.
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.
When Ed breaks his arm, his forearm is suddenly longer than it's supposed to be.
The TV shown in the background in the Denborough 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.
In the June section, Lethal Weapon 2 is advertised on the marquee of the Capitol. However, the film was not released in theaters until July 7, 1989.
While Bill's poster for Beetlejuice looks similar to the original one-sheet, it is a reprint from several years later that does not have the same credits layout as what would have been available in 1988.
When the town is having a parade and the children are talking within their group the baton twirlers in the background disappear and reappear from cut to cut.
When Beverly is in the bathroom stall and the female bullies are harassing her, one of the females pours the liquid trash from the trash bag onto Beverly. Beverly swings her backpack above her head, catching most of the liquid trash with her backpack. A bit later when she meets Ben for the first time outside of the school, you can clearly see that her backpack is in perfect condition. No sign of any trash or liquid being on her backpack.
During the final scene between Beverly and Bill, Beverly places her bloody hand on Bill's right cheek. The blood smear on Bill's cheek changes between shots.
A US Postal Service Jeep is seen featuring the "Sonic Eagle" logo in 1988. The "Sonic Eagle" logo was introduced in 1993.
When Beverly smokes in the bathroom stall, you see her put out the cigarette on the wall with her right hand. In the next shot, seen from above, you can see her putting it out with her left hand, and in the following shot, she does it with her right hand again.
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.
The movie theater says "Lethal Weapon 2" is playing on July 4, 1989, but the movie didn't come out until July 7, 1989.
During the scene in the drug store, you can see Covidien Curity bandages on the shelf. The scene takes place in 1988. Covidien is a medical product manufacturing company that was known as The Kendall Company back in 1988. It was not named Covidien until 2007ish, products were not re-branded/repackaged until 2009-2010.
There is a brief shot of some snare drummers in a marching band in 1989. They are playing on Yamaha SFZ drums which did not debut until 1992.
Richie Tozier crosses his legs when Beverley Marsh breaks the boys stare whilst she is sunbathing, in the next cut he is sat without his legs crossed.
When Ritchie insults Henry he mentions his mullet. While mullets as a haircut were certainly popular in 1989, they weren't known as "mullets" until the mid-90s. Most sources agree that the first common usage of the term was by the Beastie Boys in 1994 on the song "Mullet Head". Originally coined in 1967 in the movie Cool Hand Luke.
Ben has a visible graze on his chin after being beaten up by Henry, but the day after the graze has completely disappeared.
Despite having cuts in their hands, mike holds hands and when they separate hands there is no blood.
When Stanley is reading in the synagogue and closes the book, the book was upside down as can be seen by the Hebrew letters.
Just after the kids get out of school in June 1989, Lethal Weapon 2 is showing at the town movie theater. Lethal Weapon 2 didn't come out until July 7.
The blood on the neck/face changes in the final fight.
The drum equipment (Yamaha SFZ snare drum) used by the marching band wasn't introduced by Yamaha until 1992.
The free floating snare drums used by the high school band weren't widely popular until the 1990's. Some of the Drum Corps may have used similar drums maybe as early as 1989, but not a high school.
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 it's 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.
When Bill holds up a Lego turtle made by Georgie, it is partially made out of lime green Lego bricks. However, lime green Lego bricks were not produced until 1993.
When Beverly Marsh is buying tampons at the drugstore when she runs into the boys in the middle isles none of the products are from the 80's fore example, Olay and Garnier eye creams.
The local theater is showing Lethal Weapon 2 when the kids get out of school for the summer in June. At the same time, there are posters for the upcoming Fourth of July celebration. Lethal Weapon 2 opened on July 7, 1989.
When Beverley takes her dad's measuring tape to explore her bathroom sink's drain, she drops the tape in the sink when she is being wrapped up by the gunk shooting from the drain. After the blood explodes from the drain and coats the bathroom, there is a shot of the the bloody sink but the tape is not there. The shot is at a bit of an angle, but the tape should definitely be visible in the sink as shown.