Back to the Future is often hailed as one of the best-written movies ever produced. Time travel is notoriously tricky to navigate in film, but the 1985 blockbuster manages to do so while delivering smart jokes, satisfying pay-offs, and fully developed characters. Despite its many successes, the movie contains at least one potential flaw that's been bugging viewers since its debut: Why do George and Lorraine McFly fail to realize that Calvin Klein and their son Marty are the same person?

According to The Hollywood Reporter, Guardians of the Galaxy director James Gunn recently pointed out this plot hole while discussing what makes a perfect movie on Twitter. "A perfect film can be different from a favorite film, or a great film," he wrote. "Back to the Future SEEMINGLY could be imperfect (why don't Mom and Dad remember Marty?), but I would still argue it's a perfect film because there are reasons why this could conceivably be the case (time protects itself from unraveling, etc). Or maybe I'm in denial. Who knows."

When Marty travels back to the 1950s, where he stops his mom and dad from meeting but subsequently gets them to fall in love, his parents don't recognize him because he hasn't been born yet. At the end of movie, Marty returns to a new present-day 1985 that's been shaped by his actions in the past. That means, according to some critical viewers, that George and Lorraine would remember the person who introduced them in high school and thus realize he looked identical to their teenage son.

But this plot hole may not be much of a plot hole at all. The Hollywood Reporter gave Back to the Future screenwriter Bob Gale a chance to defend the script, and he said it isn't a stretch to think the McFlys wouldn't connect their son to someone they knew for a week in high school. "Bear in mind that George and Lorraine only knew Marty/Calvin for eight days when they were 17, and they did not even see him every one of those eight days," he told the publication. "So Lorraine and George might think it funny that they once actually met someone named Calvin Klein, and even if they thought their son at age 16 or 17 had some resemblance to him, it wouldn't be a big deal. I'd bet most of us could look through our high school yearbooks and find photos of our teenaged classmates that bear some resemblance to our children."

Gale's statement proves that the screenplay for Back to the Future is hard to pick apart. The movie is filled with details you may have missed on your first viewing—like the deeper meaning behind a famous line spoken toward the climax. Here are some more things to look for the next time you watch the movie.

[h/t The Hollywood Reporter]