Prince William and Kate Middleton shocked royal fans with a daring move.

The couple accepted a mysterious gift from a fan during a walkabout.

The Prince and Princess of Wales greeted the public at a Coronation Big Lunch.

William and Kate mingled with the public during the unplanned walkabout.

The Prince of Wales sipped from a cup containing a mysterious drink, breaking royal protocol.

Kate also tasted the drink before the couple shared their experience on Instagram.

Media reports suggest that the royal protocol is in place to prevent poison attempts.

The couple's move has left fans in disbelief and admiration.

It's a fascinating story of royalty breaking the rules and embracing spontaneity.