Many years ago I was fortunate to be invited to fly on the flightdeck of a DanAir 727 from Gatwick to Ibiza, the plan was to fly out full and come back empty but as it happened the Monarch flight parked next to us at Ibiza was overbooked and 2 pax would have to be left behind. The Dans captain was asked if he would be willing to carry them and he said yes, so these 2 pax jumped on board and had the whole cabin to themselves.