Princess Beatrice cut open Ed Sheeran's face in a mock-knighting of James Blunt that went wrong, it has been reported.
At a party at the Royal Lodge in Windsor, the home of the Princess' father, Prince Andrew, Blunt is said to have joked that he would love to become Sir James.
The Princess obligingly picked up a sword, and Blunt, the musician, best known for his 2004 hit You're Beautiful, got down on one knee.
But, the Sun reported, when she lifted the sword in order to tap him lightly on the shoulder, she accidentally swung it back, catching Sheeran, also a singer-songwriter, on the cheek.
"The blade cut into his face and it was just a few inches from his right eye," a source told the newspaper. "A couple of the guests said it could have been worse and he could have been blinded in the eye, but Ed really played it down.
"Everyone was totally shocked and apparently Beatrice was very upset. But Ed was the perfect gentleman and told her it was just an accident.
"However, it became apparent that the wound needed treatment, so Ed was taken to hospital. He had to have stitches put in."
Sheeran, attending the party with his girlfriend, Cherry Seaborn, was treated at a nearby hospital before returning to the party.
The pop star behind the Grammy Award-winning song Thinking Out Loud was well enough to perform at a concert in London the following evening.
Around 20 people were said to be at the event, including the Princess' mother, the Duchess of York.
Prince Andrew, the Duchess' former husband, was away on a trade mission.