Michael Carpenter / WENN 1 / 8
Michael Carpenter / WENN 1 / 8

Oops, she did it again: Britney Spears is back to behaving badly!

An insider says "The X Factor" creator Simon Cowell is already anxious about Spears' unpredictable behavior. "He wonders if hiring her was a mistake," a source tells Us Weekly.

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The trouble began on May 24 at auditions in Austin. "When a contestant botched her hit 'Hold It Against Me,' she just left," says a witness. "She missed the next four auditions. People weren't sure if she was coming back." (Spears, 30, later tweeted, "LOL was just taking a little break people.")

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Another issue? Her judging style.

A witness says that while Cowell, 52, L.A. Reid, 56, and Demi Lovato, 19, critiqued contestants, Spears "always said nice things, even telling one mediocre singer he could be 'bigger than Justin Bieber.' Simon was shocked!"

While an "X Factor" insider counters, "Simon doesn't regret hiring Britney, and as a judge, she had strong opinions," the first source cautions, "I don't know how she's going to handle the live shows!"

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For more on Cowell's concerns over Spears, watch this week's Hot Stuff video, as told by Senior Editor Ian Drew and special guest, Bravo star Tabatha Coffey!