During the party scenes Jodie Foster's hair changes. 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" At the end of the film in the scene where Jeanie visits Annie's grave, the grave stone reads "Annie Malick 1964-1979". Those years of birth and death are equal to age 15.