Shortly before 1 a.m. on November 18, Britney Spears entered the X-rated Hustler Store in West Hollywood.

Spears loaded up on naughty skivvies and headed to the fitting rooms. But store employees "told her they don't allow people to try on underwear," a source at the scene says. "She was really upset."

Adds the source: "She looked out of it. There was nothing going on behind her eyes."

At that point, Spears threw a fit, and took off her own underwear before trying on a pair of boyshorts (with "Barely Legal' stitched across the rear end) in the middle of the store while 15 other customers looked on.

An eyewitness tells Us, "The employees kept saying 'Don't change out here!' She's just like, 'Well, I couldn't take them in the fitting room!' It was like dealing with a child."

Spears' tantrum only continued. "The staff told her she had to pay, and she rolled her eyes, but paid with a credit card," the source tells Us. As payback, "on her way out, she went up to a mannequin, snatched the wig off the head, and stole it!"

Us Mag