Appearing on a US talk show, the girl wizard was forced to put up with some good-natured ribbing when the host pulled out a picture of her inadvertently revealing her smalls in public.
She laughed off the incident on U.S. television, saying: "At least I was wearing underwear".
The error happened as Emma went to gather up the skirt of her vintage wraparound dress to avoid stepping on the hem at the London premiere, giving the paparazzi a shot of her undies.
"Tell us what's going on here," deadpanned chat show king David Letterman as he put a copy of the picture on his desk.
"This was a small wardrobe malfunction that happened" replied the actress blushing slightly and adding: "I'm still learning!".
Meanwhile, her co-star Daniel had his own embarrassing situation to deal with when he was persuaded into a series of disguises to impersonate US personalities.
The 19–year-old was appearing on MTV's The Alexa Chung show when the British presenter told him: "Harry Potter always has the same old look - I decided that perhaps you needed a make-over".
Daniel gamely tried on a pair of shades like those worn by hip-hop star Kanye West to become 'rapper Potter'.
He then grabbed an American flag to imitate Alaskan politician Sarah Palin, joking: "I can see Hogwarts from my house" – a reference to her election campaign statement that she was experienced in foreign affairs because Russia can be seen from her state.
Finally, the young wizard earned a round of applause from the audience as he pulled on a pair of goggles to mimic Olympic swimming champion Michael Phelps.