Think Justin Bieber has reformed his crazy party boy ways? Think again!

According to model and Playboy Bunny Sarah Harris, the 21-year-old pop star got a little too touchy-feely with her during a party at Hugh Hefner's mansion in August.

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Sarah claims it all started when a fellow Bunny got mad at the Biebs for refusing to take a photo with her.

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"[After it happened], I said to her, 'Look babe, we've kind of been taking photos all night and we're over it — imagine how he feels, he gets that every single day," the blonde beauty explained to The Edge 96.ONE's Mike E & Emma.

But after Justin gave her a hug and a kiss as a thank you for defending him, "He grabbed my boob!" Sarah revealed. "And it was my instant reaction to just backhand him across the face."

Oh my! We would have loved to have witnessed that!