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.
Revealing mistake: When they're having a rock war, Henry throws a rock at Beverly. When the rock hits her, it bounces right off her, making it obvious that it was a rubber or bouncy ball, and not a rock.
Revealing mistake: When Henry is about to carve his name into Ben, he kicks Henry and falls back through the fence. As he is rolling his hair is darker than usual, making it obvious that it was a stuntman.