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Justin Bieber

Kevin Mazur/Getty Images

We can get down with Bieber Fever.

While Justin Bieber was out shopping on Mother's Day at the Grove with—shocker—his mom, the most hysterical part about the fan sighting was what you didn't see on camera.

Like how he really makes the girls within a 20 feet radius utterly squirm...It's so friggin' Hollywood.

One twentysomething babe who saw the teen sensation in Forever 21 fills us in on the scene:

"I wasn't sure if it was actually Justin, because he is so small. I mean supertiny. I couldn't tell if it was him or not. Then I noticed a few teenagers, probably about 13 years old, on their phones snapping photos and text messaging away. I asked them 'Hey is that Justin Bieber?' and one of the girls looks up, straight faced, with her cell in hand and goes 'Duh, that's why we're, like, freaking out.' "

From all the stories you hear about J.B. mania, like how he had to have his Australian concert shut down due to safety concerns and had eager fans start a mall riot in Long Island, leave it to L.A. teens to consider calmly texting their friends while the heartthrob of their dreams is only a few feet away "freaking out." Que American.

Our source chatted briefly with The Biebs on the way out of the store and says Justin "couldn't have been nicer."

Still, not all girls were quite as calm, but still pretty cool. As our Grove source made her way to her car in the garage, you could hear "Justin Bieber! Justin Bieber!" being screamed from afar. No reports of mob rushes, so hope the singing ministud escaped unscathed! Robert Pattinson should only be so lucky.

Making a less successful outing in L.A. was...

Mischa Barton, at the Missoni store on Rodeo Drive for a party celebrating author Derek Blasberg's trendy how-to guide of sorts, Classy: Be a Lady Not a Tramp. Did Meesh pick up a copy? Hmmm...

Judging by the O.C. star's recent shenanigans, one would hope so.

Of course, M.B. tramped it up for the outing, showing up very late and clearly not dressed to impress. "She looked like she picked her dress up from the Ross clearance bin," a fellow partygoer gabs about Mischa's fashion faux pas.

Another guest who made the unfab list (at least when it came to wardrobe) at Missoni was...

Kate Bosworth, who had her hair slicked back and donned a supershort kinda blah gray number. As for boy-toy Alexander Skarsgard, who is seldom seen without the blond babe?

"No vampire arm candy in sight," our source confirms, though with a tinge of sadness. Us too, doll—we love a good A.Skars sighting. Even if it's more on the put-on side!

—Additional reporting by Taryn Ryder and Susan Michals