timtan - As I said above, airlines deliberately overbook the plane because from previous statistical data they know that a certain percentage of passengers are highly unlikely to turn up. The amount they overbook will vary depending on the time of year, the route, historical data etc, etc. It's perfectly legal and happens all over the world.

They balance the risk between the lost revenue from having empty seats on the plane and the compensation they have to pay out in the event that more people turn up than can actually fly.

I'm surprised that people (who I assume have flown before) are not aware of this.