The Back to the Future series was full of time-travel mishaps (for example, Twin Pines Mall becomes Lone Pine Mall after Marty mows down a tree). But none are as eerie as this scene, which played out in both Back to the Future I and II.
Why is it eerie? Because even though there's a shot-by-shot similarity—something you can expect from a series about time travel—the character of Marty's girlfriend Jennifer in the first movie has been swapped out by an equally doting doppelganger.
It's as if the movie were saying, "Time will replace you with someone else ...and no one will even notice."
Actually, actress Claudia Wells turned down the sequels because her mother had cancer. Elisabeth Shue took on the role for the second and third movies.