About 10 years ago I was booked on LGW-FRA, cheap ET, and accidentally went to LHR instead (so used to flying from LHR and didn't consult the ticket...)

Arrived at the (wrong) airport in plenty of time. I realised my mistake as I walked into the terminal and checked the flight number. Straight to the BA Gold sales desk where the agent said that although it was a non-exchangeable ticket, as it was obviously a mistake and I was Gold he'd check me on to an LHR-FRA leaving shortly.

(Ah, the days before mobile phones! As a result, I arrived at FRA about an hour before the LGW flight. I duly turned up at the house of the friend I was visiting, and you should have seen the expression on his face; he'd checked the flight arrival times on the German equivalent of Ceefax to see when to expect me, and worked out that I must have driven at over 200mph on the autobahn in the rental car to be there so early...)