When Fred goes to ask Judy to sing with him, and talks to Kevin, Kevin's hat is forward in close-ups, and backwards in far shots.
When Fred and Bertha make the "Party" video, they put cotton in pants to make fake legs. But when the finished video is shown, the fake legs are now full-body dummies.
Kevin's cast disappears and reappears throughout the film.
When Fred goes crazy in his house after discovering that Judy moved, the lamp on the left changes from a small lamp to a big lamp. However, this could have been done deliberately for whatever reason.
When Fred uses the trampoline to get over the wall, its an obvious stunt double, with different colored hair.