IDOL: Simon & Kara Spar

American Idol judges Kara DioGuardi and Simon Cowell got under one another's skin during Tuesday's show, as the tension between the two judges was evident to anyone watching.

The straw that broke the camel's back Tuesday came after contestant Kris Allen's rendition of One Republic's Apologize, which DioGuardi and judge Randy Jackson selected for the Arkansas native to sing on the important night.

DioGuardi expressing her disappointment with Allen's interpretation of the song, which irked Cowell, who pointed out her flaw in reasoning once it was his chance to speak.

"It's a copout," he said. "You can't choose a song for him then blame him for doing the song. You didn't hold up to your responsibility."

Naturally, being undermined on national TV had the first year judge seeing red.

"You're gonna tell me about interpreting songs?" DioGuardi said. "Have you ever interpreted a song in your life?"

DioGuardi, toward the end of the segment, did the only this she could to shut Cowell up -- placing her hand over his mouth (This was not the only time this occurred on the touch-feely night, as Cowell also held his hand over a yipping Paula Abdul's mouth).

American Idol's results show airs Wednesday at 9/8c.