LHR-SIN. Comfortably cocooned in my club world seat, late in the boarding process a gentleman arrived looking very confused. "I think you're in my seat" he said, showing me a boarding pass for the exact same seat.

"That's weird" I responded, showing him my boarding pass for the same seat. Crew member arrived to adjudicate, and inform me that I was in the wrong...
...and my correct seat was 1F in First. Not something I felt any need to argue with.

It appeared that I benefited from an upgrade so late in the process, that I had already boarded when it was applied.

That's a hit rate of 3 out of 10 of my most recent long-haul flights. Possibly a combo of GGL and full-fare J tickets puts me high up the list.