Britney Spears Has Vegas Nip Slip

Britney Spears revealed herself to fans in Vegas when her leotard shifted around her chest ... but powered through it like a champ.

The nip slip went down Wednesday night at Planet Hollywood, where she made a hard right turn during a strut ... causing her left boob to pop out.

Ever the professional, she paid the wardrobe malfunction no mind for at least a good minute. Probably took a cue from Janet ... don't freak out.