Corrected entry: At the beginning of the movie, they are leaving High School for summer break. However, all of the kids are 13. (For instance, Stan is having his Bar Mitzvah, and Ben notes later on that he will be 40 the next time IT comes around.) However, most kids start High School when they are 14 or 15. That all the kids are 13 at the end of High School is a glaring mistake, or they are all advanced, which I doubt, as Eddie does not know the difference between "placebo" and "gazebo."
Corrected entry: When the postcard gets splattered with blood, she never cleans it off. When the kids are cleaning the bathroom, the kid who gave it to her goes in her room and sees it. No blood on the postcard. He would be able to see the blood because he is a child. When her dad shows her the card before she kills him, it has blood on it from her point of view.
Corrected entry: When Bev is in the school bathroom, bullies poor liquid garbage on her - she puts the backpack over her head and the backpack catches most of it. Later when she goes outside to talk to Ben, the backpack looks clean and new.
Corrected entry: They are listening to Bust A Move by Young MC in the quarry scene, a full year before it was released in 1989.
Corrected entry: The main characters all go into the well house, when Eddie breaks his arm. Whenever you see the well house usually, it has the dirt road outside with an empty lot over the street with a rusted out car. When they run out of it after Eddie's arm breaks, his mother pulls up right outside and the street is a proper tarmac road with houses on the opposite side of it.