In a video, Cowell, joined by other judges Amanda Holden, Alesha Dixon, and David Walliams, were watching a performance of "The Impossible Dream" by Welsh opera singers Richard and Adam Johnson when suddenly, a woman raced onstage and started pelting the judging panel with eggs.
The woman was quickly rushed off stage by security and the performance continued without a hitch. At the end, the judges gave the brothers a standing ovation and Cowell, 53, is seen taking off his jacket, presumably because it was covered in egg yolk.
The egg thrower was later revealed to be 30-year-old viola player, Natalie Holt, who was onstage playing with the orchestra at the time.
"I want to apologize to Richard and Adam for overshadowing their performance," she said in a statement. "I’ve never done anything like this before and in hindsight I have realized it was a silly thing to do."
Cowell seemed to take the incident in stride, tweeting after the show, "I really don’t like eggs."