It seems odd that gate agents and the Passenger Service System making all the beeps as boarding passes are scanned can't manage the count, but I've certainly seen incidents where number of passengers thru the door exceeds number of seats on the aircraft. Some people get bumped. The plane won't fly until everyone is seated. Contracts of Carriage identify bumping priority - that's a DOT requirement, and it's a DOT requirement that the stated procedure and priority be followed. I've never seen anybody dragged off - very few times have I seen Security/LEO board the plane to remove a bumped passenger.