Jennifer Aniston gets mooned during interview. Was it Gerard Butler?

During Jennifer Aniston's chat with Access Hollywood's Billy Bush at her Mexican birthday bash, one of her pals mooned her to crack her up.

Was it Courteney Cox, David Arquette, Sheryl Crow or Gerard Butler? 

You may find out if you watch the multi-part interview on Access this week. But we're putting our money on that crazy "Cougar Town" Courteney.

Aniston talks about dealing with the paparazzi photographing them on boats. And Billy even dared to ask about her "The Bounty Hunter" costar. 

"We just had so much fun together you know. He's just fun. Gerry's the most unpretentious guy.... he's absolutely the most lovely and sort of self-deprecating and just what you see is what you get. We look at work the same way...we always had each other's back."

Just FYI, Jen rented a $9,000-a-night, four-bedroom villa at the lush exclusive resort, the One&Only Palmilla Cabo. And Butler stayed in a one-bedroom casita just a stroll away.

But that means nothing. John Mayer also stayed in a separate casita and look what happened.