Fans of the wizardly franchise spotted a huge mistake in the 20th anniversary reunion over the weekend when a photo of actress Emma Roberts was shown instead of Harry Potter star Emma Watson.

Watson, who played Hermione Grainger, reunited with co-stars including Daniel Radcliffe and Rupert Grint for a trip down memory lane alongside a number of their famous co-stars and crew.

During Harry Potter 20th Anniversary: Return to Hogwarts, which debuted on Sky and NOW TV on Saturday, a picture of a very young 'Emma' wearing Minnie Mouse ears was shown.

However, the photo was in fact of Emma Roberts – best known for her roles in We're The Millers, Scream Queens and American Horror Story.

Fans were quick to spot the mistake and even discovered that Roberts had shared the picture herself in 2012.

This wasn't the only error in the reunion special as actor Oliver Phelps, who played George Weasley, pointed out.

Sharing a picture on social media, Phelps explained that Potter bosses had got him and his brother James mixed up and were given the wrong on-screen credits.

Phelps wrote: "I guess after all those pranks over the years somebody decided to get their revenge."

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A statement from HBO shared with read: "Well spotted Harry Potter fans!

"You brought an editing mistake of a mislabeled photograph to our attention. New version up shortly."