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.
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.