The 25-year-old - who looked elegant in a floor-length off-white dress - was de-robed under the camera flashes at the Les Misérables premiere in London.
Showing off her black underwear and braless top half, the singer seemed unaware of exactly how sheer the dress was at the front.
But she wasn't the only one to reveal a lot of flesh as actress Isla Fisher turned heads in a tight-fitting, sculpted, strapless gown with cut-out panels.
Anne Hathaway went for old-school Hollywood glamour in a pearl-studded Givenchy gown.
Co-star Amanda Seyfried stood out in a flamenco-inspired monochrome Valencia dress.
Model Laura Bailey left an impression on the red carpet in a pink Valentino gown, while she also appeared to have forgotten to cover up her top half with underwear.
She tweeted: "I don't care if it's not black-tie, or if it's better to be under-dressed... Tonight I am wearing a gown. And diamonds. #lesmis."
She joined stars Hathaway and Russell Crowe at celebrations well into the night, writing: "After-after-party. Anne Hathaway and Russell Crowe live and acoustic. #lesmis"
Les Misérables premiere: