Which was when Will got a telling-off from the Queen.
Thankfully, the sweet moment was captured for all to see.
Journalist Brandon McGinley shared the clip on Twitter, writing, "Come for the Queen scolding William, stay for the George facepalm."
Of course, as any of us would do after receiving a telling-off from grandma, especially if she's the Queen, Prince William appears to stand up straight quickly and move back into his position.
This isn't the first time William has been told off by Her Majesty.
During an interview for a documentary called The Queen At 90, the Prince remembered an incident that landed him in hot water.
He said: "We were chasing Zara around who was on a go-cart, and Peter and I managed to herd Zara into a lamppost and the lamppost came down and nearly squashed her and I remember my grandmother being the first person out at Balmoral, running across the lawn in her kilt."
He smiled and added: "She came charging over and gave us the most almighty b******ing, and that sort of stuck in my mind from that moment on."