Justin Bieber -- Dresses Down 'Today' Director ... They Can't See Me Dance!!!

Justin Bieber violated TV Rule 101 -- always assume your mic is open.

The Biebs got pissed off at the "Today" director -- during his performance of "What Do You Mean?" -- for shooting him so close, viewers never saw his dance moves.

As Matt Lauer throws to commercial, Justin is busy schooling the director. It must have caused a commotion in the control room, because the video went to black but Bieber's mic stayed open as he continued his mini-rant.