Credit: Michael Becker/FOX

"I've always been brutally honest," American Idol guest judge Katy Perry warned on Tuesday's show as Los Angeles auditions kicked off.

But it seemed that she packed the best punches for fellow judge Kara DioGuardi - not the contestants.

The two had contrasting opinions about wannabe singer Chris Golightly, who performed the classic "Stand by Me" after telling the host Ryan Seacrest about his tough childhood.

DioGuardi, 39, was eager to send him to the next round. "Chris, you are one of my favorites today, if not my favorite ... I think we may look back at this audition and go, 'Wow,'" she told him. "You're the kind of kid who has just enough talent and just enough of a story and pain and stuff that you've gone through in your life to really connect with it, and I think you're only going to get better."

But Perry, 25, didn't agree with her and flatly said "no" on advancing him.

"This is not a Lifetime movie, sweetheart," Perry sniped to DioGuardi. "You have to have talent!"

Simon Cowell agreed with Perry, prompting DioGuardi to complain, "I have Simon and Katy ganging up on me!"

But the real zinger came when DioGuardi began belting out Perry's smash hit "I Kissed a Girl." Within seconds, Perry interrupted, "Please stop before I throw my Coke in your face!"

DioGuardi tried to played down their differences. "We used to be friends," she said during the show. "We were, like, cool."