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.
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.
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.
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.
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 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.
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.
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 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.
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 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.
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.
When Bev sits in the tub looking at the poem on the post card, she sets it down with the writing facing up. Later, not only does the position of the card change, but it now has the photo side facing up. Also, the card would have been warped after being covered in such a mess. Later we see only minimal streaks on the card.
After Patrick gets spooked by the Barron ghosts, he runs away and falls face first into the gray water. When he gets up, his face is wet, but relatively clean. When we get a clear shot of his face again when he's trapped, his face is covered in blood.
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.
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.