Movie spans in '74-'75, and early they show a joint being rolled on a copy of Hunter S. Thompson's "Fear and Loathing in Las Vegas", with the tagline at the bottom (of the book) "From the author of the Great Shark Hunt", which was published in '79.
In Dunph's first day of school, he is in a math class where the teacher calls on him to solve a problem. The problem is written on the blackboard behind him. Later on in the film, what should be months have passed, and back in the classroom, the blackboard remains completely unchanged.
In the scene in the hockey rink, the markings on the ice are from the 1990s. The most noticeable is the blue circular goalie crease. In 1974, the goalie crease was rectangular and white.