Kris Jenner is opening up about her most embarrassing moment — and let's just say, it's a doozy (and involves Caitlyn Jenner, 65)!

"The most embarrassing moment for me was when I was with [Caitlyn] on a commercial airliner in first class. We joined the Mile High Club and we felt we got away with it," the 59-year-old told Cosmopolitan.

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"We had sex in the bathroom and we came out and nobody said anything. At the end of the flight, the flight attendant got on the microphone. 'Congratulations, Mr. and Mrs. Jenner! You've just joined the Mile High Club. We're so proud of you, and we decided to give you a bottle of champagne! Yay!' I could not squish down in my seat low enough. I was mortified!"

The momager's confession is part of the Kardashian family's Cosmo spread, in honor of the mag's 50th anniversary.

Check out more of the Mile High Club's most famous members —>