Jack Osbourne was accidentally given alcohol at Princess Beatrice's 18th birthday party.
The 20-year-old TV star, the son of Ozzy Osbourne, had not touched a drop of alcohol for three years since he went to rehab to be treated for alcoholism and drug addiction.
However, at the lavish bash, held at Windsor Castle, Jack accidentally knocked back a vodka jelly without realizing it was alcohol.
He revealed to Britain's Closer magazine, "They brought out a strawberry jelly and didn't say it had vodka in it!" Jack and his sister Kelly were among the 500 guests invited to the 1888-themed masked ball, which cost a whopping £400,000.
Other celebrity guests included Demi Moore and her husband Ashton Kutcher and Peaches and Pixie Geldof.
Jack went dressed up as notorious Victorian serial killer Jack The Ripper, but claims he didn't enjoy the party as he found it too posh.
He said, "The amount of posh people there was unreal. There were so many Eton schoolboys. I couldn't relate to them. I don't walk around wearing a ring with my family's crest on it, saying, 'Woof woof!'"