During a flashback to summer of 1989, an arcade cabinet of "Mortal Kombat" is clearly seen at the movie theater. But the game was not released until October of 1992. A cabinet of ''Mortal Kombat II'' is also visible in the arcade, while the game actually came out in 1993.
When Bill decides to buy back his old bike, he is talking to the seller (Stephen King) for quite some time. If you pay attention, you can see that the book on the counter has been rotated several times between the cuts.
Beverly is kidnapped by Pennywise towards the end of the first movie, prompting the other boys to go to Neibolt to save her. In the flashback to the aftermath in this movie, Beverly is on her bike. She wouldn't have had her bike if she'd been kidnapped by Pennywise.
when the older losers look in the window and see their younger selves, the scene flashes back to the younger versions talking after defeating IT. Eddie is still covered in vomit but Stanley doesn't have the wounds on his face from being bitten by Judith.
After the climax, the camera pans over the town and you see the sign "Royal Lepage" on one of the buildings. Royal Lepage is a Canadian real estate owner-operator that doesn't exist in the US, so you know it wasn't filmed in Maine.
When Richie has a flashback at arcade saloon, he likes playing Street Fighter, which came out at 1987, beside that cabinet there is Mortal Kombat II at the left, assuming that is 27 years ago, then Mortal Kombat II was not released until 1993.