Britney captioned the photo with, "Fancy running into this guy! Such a sweetheart!"
Ed O'Neill didn't recognise Britney Spears when she asked him for a photo in an airport lounge.
The 70-year-old Modern Family star shared the embarrassing story on Ellen.
"I was at LAX," he said. "I was flying alone to Hawaii and I was waiting for my flight to board."
Just as his flight was called, O'Neill saw a woman approaching but he had no idea that it was Spears.
"She came up and said, 'Oh Mr O'Neill, I love Modern Family and you're my favourite on the show'."
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The 34-year-old singer "couldn't have been nicer" according to O'Neill and he happily agreed to pose for a photo, which Spears later shared on social media.
"We took it and I said, 'have a nice trip' ... and I left'," O'Neill said.
"The next day my manager text me and he said, 'What is this? It has 53,000 likes. It's Britney Spears!'
"I didn't know it was her," O'Neill admitted.
The actor, who plays Jay Pritchett on Modern Family, was so embarrassed that he later called Spears' manager to apologise.
The Australian site cited a 'heightened threat level' as the issue.