I often look at the Heathrow Arrivals/Departures board to get an idea of when a particular aircraft might move and it is surprising how often the departure board will announce "Departed. Airborne at xx:xx" when the aircraft is obviously still at the gate preparing to move.
If that's true, it's a scary thought, given that ATC are responsible for logging the "Airborne" time and putting it into the system. Hopefully that doesn't mean they are giving takeoff clearance to the wrong aircraft.
Having said that, I've never seen "Departed" on Heathrow's flight status page - the normal sequence of events posted is "Last Call", "Gate Closed", "Taxied" and "Airborne hh:mm".