Many details in Back to the Future were thought through very carefully, with even the smallest moments throughout the trilogy worth notice due to the consideration put into them. These include little details, such as Doc wearing a bandanna in Back to the Future Part III made from a shirt he had previously worn to a "Smoking Required" sign in front of the Biff Tannen Museum in Part II.

However, even with so much attention to detail, there are still unexplained Back to the Future's time travel rules or with something completely unrelated. Some Back to the Future plot holes and questions raised, however, have nothing to do with time travel and more to do with issues surrounding Marty's family, like whether his father ever figured out that "Calvin" was a time-traveling Marty. One plot hole still has heads scratching, though.

Lea Thompson Played Marty's Mom And Paternal Great-Great-Grandmother

Lorraine and Maggie Look Alike But Are Not Related

One of Back to the Future's biggest plot holes is why Maggie McFly looks so much like Marty's mother, Lorraine. Both characters were played by Lea Thompson, but Maggie McFly was related to Marty through his father rather than his mother. Maggie looking so much like Lorraine doesn't make sense; Lorraine's maiden name was Baines, and they are not related. Yet, they look pretty much identical.

However, Back to the Future writers Bob Gale and Robert Zemeckis did consider this incongruity when they made the decision to have Thompson play Maggie McFly in Back to the Future Part III. They explain in an answer to a fan question in the Back to the Future FAQ that there's one very simple, satisfying explanation: men in the McFly family have a type, and they just happen to be attracted to women who resemble each other. Gale and Zemeckis point out that even Jennifer has physical similarities to Marty's mother and great-great-grandmother (via Back to the Future FAQ).

Why Lea Thompson Was Cast As Maggie McFly After Playing Lorraine

They Couldn't Resist Repeating The Callback To Earlier Back To The Future Movies

It would have been a shame if Back to the Future Part III broke the trend of Lea Thompson's inclusion, and those involved with creating Back to the Future agreed. Every Back to the Future movie has a scene where Marty wakes up to a Lea Thompson character, and if Thompson hadn't played Maggie McFly, the absence of such a scene in Back to the Future Part III would have been noticeable.

Thompson isn't the only Back to the Future actor to play multiple characters, and while she's the only actor to play two characters who are technically unrelated, it might have stood out as odd if they hadn't brought her back as Maggie. Even though Thompson playing Maggie McFly creates a plot hole, it's worth having her in the film, since the repetition of certain elements throughout the Back to the Future films is such an important detail of the trilogy.

In a way, Thompson's role as Maggie McFly even adds a layer to the first Back to the Future film. Since the romantic relationships of the McFly family are so important to Back to the Future, continuing the trend of McFly men falling in love with women with similar appearances gives that situation more depth.

Source: Back to the Future FAQ