Continuity mistake: In the opening scene the camera pulls back from a bedside locker to an over the shoulder shot of Kathleen Turner's character doing her make up in the mirror. Close inspection shows that the shoulder in the foreground is actually that of a double who doesn't quite match Turner's movements.
Revealing mistake: Last scene in the movie is Peggy Sue waking up in the hospital. The very last shot of that scene is of the camera backing up from what appears to be a mirror. If you look closely, it isn't a mirror at all, it's actually a window made to look like a mirror complete with another set of actors to mimic the stars of the movie's actions.