While she was serving as a guest judge, Clarkson shared a video of herself to Instagram as she practiced delivering some of Cowell's famous lines in her best approximation of a British accent.
As the pair watched a clip of her attempts, Cowell couldn't help but laugh and admitted with sarcastic applause, "That was dreadful."
"Let me try to do one in an American accent," Cowell said. "Give me something American to say."
"I want you to try and do Texan," Clarkson challenged him.
"Okay, I'll do Texan," Cowell said, before trying his best to say "How y'all doing? I think I want a barbecue," in something that didn't really sound like an accent from any part of America.
"Nope! Where are you from, the Bayou?" Clarkson shot back, after busting out in laughter. "Yours is about as good as my accent!"
The playful banter between the stars is a sweet example of how their relationship has evolved over the 19 years since they first met when Clarkson competed on -- and won -- the first season of American Idol in 2002.