Credit: Mark Sullivan/; Lester Cohen/

Although Sean "Diddy" Combs was a guest mentor on Wednesday's episode of American Idol, his work with the show's singers was pre-taped -- which meant there were no awkward on-camera encounters with Jennifer Lopez, his most famous ex-girlfriend.

But that didn't prevent judge Lopez, 42, from getting flustered over her former flame's appearance.

Talking about 42-year-old Combs' mentoring work, the singer took a long, uncomfortable pause when she had to speak her ex's name. Laughing, she said, "I was going to say Puffy," then dropped her head, looking embarrassed.

Now dating dancer Casper Smart, 24, Lopez dated Combs from 1999 to 2001, back when he was known as Puff Daddy, or Puffy. The former It Couple had an awkward reunion just last month at Vanity Fair's star-packed afterparty in Hollywood.

According to an observer, Lopez made a bee-line for Combs shortly after arriving. Although Lopez seemed "friendly" during their chat, she was overheard apologizing for not returning his calls and text. "It was weird," the observer adds.

Lopez, who famously dated Ben Affleck after splitting with the hip-hop mogul, has been friendly with another ex: Marc Anthony. She and her husband of seven years (they shared twins Emme and Max, 3) announced their split last July but remain colleagues on their reality show Q'Viva.