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?
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.
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.