Witnesses this last night on a flight from TXL-LHR. I upgraded to CE at check in and the cabin was a fairly light load but ET packed - the guy in the front row of ET tried to move himself forward twice - once before take off and he was immediately moved and then again straight after the seat belt sign went off - again he was immediately moved and told rather curtly 'sir - these passengers have paid a premium to sit here I can't let you stay' ! Then from behind the curtain I heard him protesting to the crew about why he had to sit in a full row when there was so many vacant seats ahead ! Glad the crew enforced it !