As the Cambridges approached one of the exhibition billboards in Waterloo, William's attention turned to the restaurant and he had a look through the window.
It was the last thing startled customers expected to see - but the Duke simply waved hello and then carried on with the engagement.
Fans were quick to share photos, and it wasn't long before KFC's social media team decided to make a cheeky joke.
They tweeted out one of the photos of him peering through the window, captioned: "William whispered quietly to himself; 'Oh, I just can't wait to be wing'."
The employee who came up with the pun later admitted he could have made another joke.
"I'm sad I didn't call him His Royal Thighness in the main tweet tbh," he said in a second tweet.
Fans loved the jokes, and were quick to reply.
"Hahaha this is brilliant! Guy just wanted a zinger box meal," one wrote.
"This is the best," another added.
Kate's exhibition, which she launched in collaboration with the National Portrait Gallery, aimed to document the fears and feelings of the UK amid the coronavirus pandemic.