KATY PERRY was performing at some video awards show in France . . . and she was caught lip-syncing.  She was singing "Roar", and the music was off . . . and it was painfully obvious that her lips didn't match the sound.  It was so bad the host walked onstage and waved his arms for her to stop.  He politely asked her if she would mind restarting.  So she did.  And for the second try, you could tell that she WAS singing, but she still didn't sound that great.