Before Harry boards the Hogwarts Express at the end of Harry Potter and the Sorcerer’s Stone (2001), Hagrid hands over a homemade scrapbook filled with photos of Harry’s parents. It’s the type of movie moment known to make viewers tear up, and even Daniel Radcliffe did while filming it—but not just from emotion.

As Digital Spy reports, producer David Heyman recently explained in an interview for Empire that Radcliffe’s green contact lenses caused an allergic reaction that made his eyes “puffy and red,” which was “quite appropriate for the scene.” It was only the first day of filming, and the allergic reaction was problematic enough that they decided to nix the contact lenses altogether. So Harry’s eyes are blue in the final cut of that scene, though they do look a little bloodshot.

The fact that Harry has blue eyes in the film series has long been a grievance among diehard fans of the books, which often mention Harry’s “bright green” eyes—the single feature he shares with his mother, Lily. Rather than ask Radcliffe to suffer through his allergy or digitally alter his eye color during post-production (which Heyman said “felt artificial”), filmmakers simply cast a blue-eyed actor, Geraldine Somerville, to play Lily. That way, Harry and his mother would still have similar eyes, even if they weren’t the right hue.

It wasn’t the only element from the books that was altered for (or totally omitted from) the film adaptations, and many Potterheads have strong opinions about what they wish had been left in. Heyman shared some of his own with Empire: Hogwarts’ resident poltergeist, Peeves; Hermione’s Society for the Promotion of Elfish Welfare (S.P.E.W.); the Quidditch World Cup game; Ron’s anxiety-ridden relationship with Quidditch in Harry Potter and the Order of the Phoenix; and the backstory of the Marauders.

[h/t Digital Spy]