EXCLUSIVE: 'Rachael vs. Guy' Turns Tense

The competition on Rachael vs. Guy: Celebrity Cook-Off is heating up, and in this exclusive clip from the hit Food Network show, Sister Act star Kathy Najimy is definitely feeling the pressure -- and possibly succumbs to it.

"I'm feeling very nervous about the Béchamel Sauce because I've never done it for 100 people before," Najimy says, while quickly whipping up the famous white sauce. "The scary ingredient for me in the sauce is the flour."

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And clearly, something goes awry when the sauce starts to take on a doughy consistency.

"How am I going to explain to Rachael [Ray] that I made a muffin?," she worries.

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Rachael vs. Guy: Celebrity Cook-Off, in which the last celeb standing (who's mentored either by Rachael Ray or Guy Fieri) wins a cash donation towards the charity of his or her choice, airs Sunday at 9 p.m. ET/PT on Food Network.