Millie is just about to play the piano; in the background you Sam is starting to tap Ken to talk to him on the couch. After Millie starts to sing, Sam disappears from the couch in the background with Ken, then Sam is seen again arriving on the couch and talking with Ken.
Just before the party Lindsay sets a bowl of chips on the piano, which has the fall board open. In the next shot, the bowl is on top of the television and the fall board is down.
When Lindsay is in math class, the teacher has an equation for the Simple Power Rule and the equation is incorrect. The -1 should be in the exponent of x so that x is raised to the n-1 power.
Neal claims that the Jewish Sabbath day is Friday, which is accurate: that's Islam. The Jewish Sabbath day is Saturday.
Nick mentions that John Bonham died the previous week, meaning that it is set in September 1980. However, a few minutes later, he talks about going to see Pink Floyd: The Wall (1982), a film that was not released in the US until August 1982.
When the guys are in the party store to buy the non-alcoholic beer, they're standing in front of a glass cooler, and it's obvious that most of the products in it bear labels and logos that weren't around until years later.
The boom mic can be seen in the shadow over the bed when Neal reveals to Lindsay that he is in love with her.