In 2016, the "E.T" singer was told by one of her friends to check Billie out but Katy was less than impressed at the time, reports Mirror.co.uk.
During a Q&A at a private event for Los Angeles based station KIIS-FM, Katy opened up about the massive fail.
She told the audience, as quoted by Mirror.co.uk, that her friend's email read: "Hey, check out this new artist. I'd really like us to work with her because she was working with me for Unsub (Records)."
"It was a song called Ocean Eyes, and it was just a blonde girl, and I was like, 'Meh, boring,'" she said.
"Big mistake, huge mistake," she added.
Since 2016, Billie has gone on to win multiple awards, become the youngest singer in history to headline Glastonbury and get a Golden Globe Award and an Oscar for her song "No Time To Die" which featured as the swan song for Daniel Craig's James Bond.