Oops! She did it again. Britney Spears was caught lip-synching by an eagle-eyed fan while  she performed the 'Britney Jean' track 'Alien' during her Las Vegas residency.

At the :06 mark of this short vid, the vocals start and Brit hasn't yet lifted her mic to her mouth. She missed her cue as she was strutting on her stage.

Most vocalists sing to a track, but that is the kinda flub that celebs hate to have caught on video by a fan.