The scenes where Pennywise' teeth are exposed as he attacks his victims were partially done by CGI. According to Arnaud Brisebois , "There are even a few shots with the kill mouth where Bill Skarsgård's face is entirely replaced," he states. "In some of the early frames, we kept his actual eyes just to create a small connection." See more

Goofs

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. See more