Originally Posted by kkirksea

On international flights you often have to show your passport with the boarding pass. If they don't match, you'll have a lot of explaining to do. Depending upon where you are in the world, you could be carted off to another room and miss your flight.

I've had my passport/boarding pass checked at the gate.. and then again right before entering the cabin, a second time.

One can also not easily move/swap while on the plane on many international flights. Many FA's monitor this like a hawk... particularly between cabin classes. ( Your mileage and experience may vary, as they say ) I remember one passenger moving from his seat to an empty row in the same cabin class... and the FA was on them, telling them that they HAVE to be in their assigned seat on take-off and landing. ( An exception is if the seat has problems and the FA then moves them.. and when they did that in two instances that I know of, the FA updated a clipboard sheet. ).