Jodie Foster's hair changes during the party scenes. In some shots, she's wearing two combs (pulling her hair off of her face), while in other shots, she's wearing one comb with the other side combed straight down.
The goof item below may give away important plot points.
In the beginning of the film, Jeanie says "Annie, you are 16 years old now." At the end of the film, in the scene where Jeanie visits Annie's grave, the gravestone reads "Annie Malick 1964-1979". Those years of birth and death are equal to age 15.