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 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.
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.
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.
A large container train is shown in the background with modern logo Hapag-Lloyd containers. While double-stacked container trains were introduced in North America in the early eighties they were not common in 1989 because of low bridges and tunnels that had to be replaced to accommodate them.
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 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.
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 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.
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.
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.
Bill's bicycle named "Silver" has an inconsistent black marking on its right side. For example, it is there at about 70 minutes into the film, but 20 minutes later it is missing, and also seems to be a slightly different bike.
While Pennywise is speaking to Eddie after just chasing him in the form of a leper, he raises his upside down triangle of red balloons. While cutting to Eddie and then back to Pennywise, you can clearly see Pennywise holding the string attached to the balloons, but, in the final shots of him speaking, the string is nowhere to be found; as if the balloons had magically disappeared. However, this isn't the case, as you hear and partially see all of the balloons popping after Pennywise has finished talking.
When Bill goes into the garage, his dad is sanding on a wood lathe facing the window, but the lathe tail stock is on the left. Typically, the tail stock is on the right. The next time we see him he is still facing the window, but the head stock is now on the left as it should be.
When Henry walks toward the red balloon on his mailbox, his pants are wet as if he peed himself (probably in fear when his father shot the gun near him). But when Henry walks into the house moments later, holding the knife, his pants are dry.
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.
When Henry kills his father, there is very little blood on his face afterwards. But when he attacks Mike later at the Neibolt House, Henry's face is splattered with dried blood. A deleted scene shows that he has slit both Belch and Victor's throats and then driven Belch's car to the Neibolt house, explaining the extra blood.