As one half of Barbenheimer, along with Greta Gerwig's Barbie, Oppenheimer is already one of the summer's biggest movies (and there are box office receipts to prove it). The historical biopic stars Cillian Murphy as the eponymous theoretical physicist, known first and foremost for leading the creation of the atomic bomb. Florence Pugh and Emily Blunt also star in the movie, as Oppenheimer's mistress and wife, respectively, in a cast that also includes the likes of Matt Damon, Robert Downey Jr., Gary Oldman, and Josh Hartnett, among others playing real-life figures.

The film, directed by Christopher Nolan, follows Oppenheimer through his research during World War II, and also covers his personal life and internal struggle over his creation of the deadly weapon. Despite the dark subject matter, audiences and critics both love the film. Rotten Tomatoes lists the film's critic score as 94%. And the "Barbenheimer" trend is only increasing the number of filmgoers interested in seeing the film, sometimes on the exact same day as Barbie.

But while some fans may be focused on the film's controversial sex scenes (yes, somehow Oppenheimer had time to have multiple affairs), others have pointed out another moment in the movie worth talking about. In the movie, there's a scene set in 1945—which has led to some viewers catching a major historical inaccuracy. While most moviegoers won't notice, those who are fans of World War II history, or just have a good eye, may realize historical detail. History buffs who have yet to see the film, beware.

So what exactly is this historical mistake made in Oppenheimer? Is it that Starbucks cup in Game of Thrones? Or is it something that messes with the plot of the film? Here's what we know about Oppenheimer and its historical issue.

What Historical Error Did Oppenheimer Make?

In a 1945 scene, we see Oppenheimer standing among a crowd. People have noticed within the scene an American flag is waving–but not the correct American flag.

Back in 1945, there were only 48 states in the United States, as Alaska and Hawaii had yet to become part of the country. According to CNN, the first flag to be flown with 50 stars was in 1960. Twitter users are pointing out the inaccuracy, and sharing that it's a widespread complaint among those who've seen the film. That being said, many still agree the film is a masterpiece despite the flaw.

But other fans argue the inclusion of the flag was intentional, as Oppenheimer actually has two timelines–one during the creation of the bomb in 1945, and the other in 1954 as the scientist is stripped of security clearance when appearing in front of the US Atomic Energy Commission due to his suspected Communist leanings.

It's possible Nolan and crew made the decision to have the 50-star flag to show the world from Oppenheimer's perspective in the far future. The scientist passed away in 1967, so he did live through the U.S. gaining those additional two states.

Whether intentional or not, we can all agree Oppenheimer is worth the watch.