Harry Potter and the Prisoner of Azkaban - Crabbe Switches Sides
In the scene when Harry, Ron, and Hermione are going to visit Higrid, they bump into Draco and his friends. After Hermione punches Draco, Draco and his friends start running up the hill with Crabbe on Draco's right. When Harry and Hermione go back in time and see themselves, Hermione explains how they've gone back in time. After that, Hermione sees herself punch Draco and Harry says "Good punch", then Hermione replies "Thanks. Draco's coming". As Draco and his friends come running up the hill, look carfully and Crabbe is on Draco's left.