If Demi Lovato doesn't pipe down when he's speaking, Simon Cowell** has vowed to put her "in the naughty corner."

The two judges -- who amiably bantered when the second season first began -- have becoming increasingly short with one another in recent episodes of The X Factor. During the Dec. 5 show, tensions reached an all-time high.

When Cowell's group Emblem3** performed a cover of Bruno Mars' "Just the Way You Are," fellow judges Britney Spears** and L.A. Reid** praised the trio. Lovato, however, told them it felt like "a downgraded version of the Jonas Brothers five years ago." Cowell then addressed Lovato directly. "That was incredibly dumb, what you just said," he sniped. "Honestly."

When the group returned later that evening to perform the Alphaville classic "Forever Young," Lovato remained unimpressed. "What is the matter with you tonight?" Cowell asked her. Throughout the night, Lovato frequently stole the acerbic Brit's microphone.

The judges did, however, make nice when Cowell's girl group, Fifth Harmony**, covered Lovato's No. 1 smash "Give Your Heart a Break."

"I think this song should have been your song," Lovato told the quintet. "It was energetic. It was fun. I think that Simon actually did a good job this week."

"Thank you, Cruella," Cowell told Lovato, 20. "I want to thank America for choosing a great song. This is what we've been trying to do to show that you are a legitimate girl group."

Us Weekly spoke to several of the remaining eight contestants post-show. "I think they have a love/hate relationship," said Carly Rose Sonenclar**, 13, who has been in the top spot for two consecutive weeks. "I think it's part of the show and people expect that now."

Country singer Tate Stevens**, 37, doesn't mind the feisty panel of judges. "You guys do your thing," he said. "Just don't take it out on me!"

Cowell, 53, has been particularly hard on Lovato's only remaining contestant, CeCe Frey**. "You're not worth a $5 million recording contract," he told the aspiring singer.

"Maybe he's just giving me a hard time. Simon just doesn't want to admit it, but he loves me," Frey, 21, told Us. "He would miss me if I was gone."

"I'm getting used to the beat down from Simon. I'd be lying if I say that it didn't get to me in some way," the Decatur, Illinois, native added. With Lovato's guidance, Frey said, "I feel like every week we are getting closer and closer to finding exactly what works for me."

The X Factor airs Wednesdays and Thursdays at 8 p.m. EST on FOX.

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This article originally appeared on Usmagazine.com: Demi Lovato, Simon Cowell Bicker on X Factor: Contestants React!