Another day, another star parties in London and lets their hair down. Or in Emma Watson's case, flashes her Britney to the paparazzi. Joining the very exclusive peek-a-boo sorority helmed by Spears, Lindsay Lohan and Paris Hilton, recently legal Hermione Granger celebrated her 18th birthday by partying across the pond with co-stars in a very demure little black dress, but made the all-too-common mistake of failing to exit her chauffered car in the proper manner. Though it appears the potential bad girl was wearing some kind of thong-ish type thing, her lacey underwear left little to the imagination. And though it's not our place, we do recommend Watson consider heading to the nearest waxer before flashing her nether regions again. A closer look after the jump.

What is it about dirty, malnutritioned, skinny-jeans-wearing rockers that turns good girls bad? The latest victim to fall prey to this time-honored tradition is none other than Miss Hermione Granger herself, Emma Watson. She was spotted leaving a party in London with Razorlight's Johnny Borrell, best known Stateside for his sordid, screaming-match-filled relationship with recent rehab enrollee Kirsten Dunst. But considering what happened to Kiki once she hooked up Borrell, we are cringing at the very thought of Emma Watson trading in her summa cum laude Hogwarts education for an unpaid and involuntary internship at the Cirque Lodge. So how do we solve a problem like Emma?