Wardrobe malfunction is a euphemism used to describe the accidental exposure of an intimate part or parts of the body due to a defect in an article or articles of clothing.
read more on wikipedia
wardrobe malfunction n. A problem with a part of one's clothing.
Keira Knightley: The British beauty had a slight dress slip at premiere of her new movie "Pride & Prejudice". The dress slipped down just enough to show off her left nipple.