Leadsled,
I remember the same thing on an Ansett flight MEL-PER back in the 80s. A guy boarded the plane at the last minute sat down in the aisle seat next to me and promptly nodded off. The feint aroma of stale alcohol permeating the immediate area. About 45 minutes into the flight when the meal service started he woke to look at the gastronomic delight now laid out on his tray table. His comment "this is pretty good for a Melbourne to Sydney flight eh"?
After composing myself, I gave the poor guy the bad news. He called one of the cabin crew and showed his boarding pass. Her comment was just as classic "yes sir, you're going to Sydney" pause "and this plane is going to Perth - oh ****".
As luck would have it, our wayward pax arrived in Perth just in time to be put on the midnight horror back to Sydney and arrive bright eyed and bushy tailed on parade next morning. He was an AJ.
At least electronic gate readers have all but eliminated this occurring these days.