Visible crew/equipment: In the scene were the adults go back into the sewer, and they have the feeling: "Something is coming", you can see the shadow of the cameraman, twice. One time, coming closer from a pipe, and another time, moving away. Look on the right wall/right on the floor.
Revealing mistake: In the library scene when Richie sees Pennywise the clown, all the balloons filled with blood are popping on people who aren't supposed to be able to see them, but one lands on a lady and you can see her jump as the balloon pops.
Pennywise: [digging graves.] Take your pick, B-b-b-billy boy. Oh, except for the one on the end. That one's already taken. Sorry!
More trivia for It
Question: What is the deal with everyone saying "beep beep" to Ritchie every time he tells a joke?
Answer: In the book it is explained that they are telling Richie to shut up.
Chosen answer: It's their way of telling Richie to be quiet. To get him to stop talking.