Whose Side Are You On? Christina Aguilera vs. ...

From The Wanted to Kelly Osbourne, see who's hit a sour note with the singer

THE WANTED

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They've since admitted regret for calling Christina Aguilera names (and her fans let them have it on Twitter!), but the Brit boy band still was left unimpressed with her congeniality after their performance on The Voice. "She's a total b–," band member Tom Parker told a New York radio station. "She just sat there and didn't speak to us. Wouldn't even look at us." Added band mate Max George, "[She was] quite rude."

ADAM LEVINE

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The ongoing rivalry between The Voice's passionate pop stars extends beyond the show. When the Maroon 5 frontman announced plans to launch his first fragrance, Aguilera took him to task on Twitter, linking to one of his prior Tweets that asked for a ban on celebrity scents. "Haha @AdamLevine," the singer wrote. "What a difference a year makes ..."

TONY LUCCA

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When the onetime Mouseketeer landed on The Voice, he thought he would find an ally in his fellow alum, but instead Aguilera consistently criticized Lucca, even calling him "one-dimensional." "I have had conversations with old friends [from The Mickey Mouse Club] and I'll ask, 'Is there something I missed?'" Lucca told PEOPLE. Since the show's conclusion, Lucca revealed Aguilera cleared the air between them, and he hopes they can move forward.

KELLY OSBOURNE

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The Fashion Police host pulled no punches when she publicly called the singer fat. "She called me fat for years," Osbourne told Glamour. "I said, 'F––– you, now you're fat, too! ... Now you know how it feels!" During a recent Watch What Happens Live appearance, however, Osbourne seemed ready to bury the hatchet. "I think she's looking way better right now," she said of her former feud mate. "She's got a great voice and she's got a beautiful son."

JUSTIN BIEBER

Kissed, then dissed? When the Biebs appeared on The Voice, Aguilera seemed to scowl at the teen star after he stopped by her chair for a quick hello. Though their brief interaction went viral, the judge defended herself on Twitter, writing, "The media loves spinning stories... Can't a girl have a little #BieberFever after getting kissed by the Biebs?"

LADY GAGA

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When Aguilera returned to the music scene in 2008, her revamped look drew comparisons to a still on-the-rise Gaga. But the "Dirrty" singer denied any similarity and even downplayed her knowledge of the star. "This person was just brought to my attention not too long ago. I'm not quite sure who this person is, to be honest," she told the Los Angeles Times. "I don’t know if it is a man or a woman." Two years later, Aguilera changed her tune, saying she is a fan of Gaga. "I think she is great," she wrote on her website, "and I appreciate any woman fearless enough to go against the norm."