At his latest '20/20' show in Philly, a fan just couldn't resist (we don't blame them) and just had to grab Justin's butt while he was performing 'Cabaret'

First, he slaps the hand away, not missing a beat, then gives a nasty stare down towards the end of the video  

Not quite as good as last year in Philly when he was flicked off by a fan and gave them the finger right back - VIDEO