Continuity mistake: Eddie's broken arm changes around, sometimes it's the left arm that's broken, sometimes it's the right.
Continuity mistake: Ben, Richie and Beverly go to the movies and after that they go down to the Barrens, where they later meet Bill and a boy named Bradley. In chapter 9, section 5, page 433 in my edition, Beverly meets Eddie, Ben and Bradley. Eddie asks about the movies she saw with Ben and Richie the day before, while Beverly recalls Bradley from having met him a week ago with Bill in the Barrens. So has it been a week or just one night?
Continuity mistake: At the reunion at the Jade of the Orient Mike shows the others scars on his chest that the werewolf gave him. However later when their venture into the house at Neibolt Street is told, and the werewolf comes for them, it never touches Mike at all.
Continuity mistake: Mike's father mentions that Henry Bowers is the worst bully at Mike's school. But Henry went to Derry Elementary and Mike to the church school at Neibolt Street.
Factual error: In chapter 18, when the Losers are in the kitchen of 29 Neibolt Street, Bill mentions that the FBI has a training range consisting of a false street with pop-up targets at Quantico. However, the scene is set in 1958, and the infamous "Hogan's Alley" at Quantico was not established until 1987.
Continuity mistake: When adult Ben goes to get a library card the librarian addresses him as Mr Hanscomb without having been told his name. Later on when she talks to Mike she doesn't know Ben's name anymore.
Plot hole: On a couple of occasions the book states that It got Victor and Belch at the same time, and that Henry only barely escaped. And when the remaining losers go down to the sewers again they find their two bodies together. Yet during the narrative of the young losers in the sewers Victor is killed early on, and Henry and Belch continue chasing the seven kids for quite some time before Belch is killed.
Continuity mistake: In the book, when Ben meets Eddie and Bill in the Barrens, they ask Ben if they know Richie and Stan. He says that he knows Richie but doesn't know Stan. Eddie then says that he probably doesn't know him because he started school late. This was because he got sick a lot when he was a kid. He also mentions that Stan just finished the fourth grade while the others just finished the fifth. But a few chapters later, when Ben meets Stan, they said that they know each other because they were in the same second grade class. So, did Stan really start school a year late, or did he start at the same time as the others? And if he did, how did Ben and Stan end up in the same second grade class? Pages 294 and 304.