This error occurs on the PGMWE1 stamp - Rowing: Mixed Coxed Four LTAMix4+ issued on 4 September.
There would have been 4 of these stamps on the A4 sheet, 16 on the press-sheet of 4 x A4 sheets. Other stamps affected would have been
PGMWE2 Athletics Men's Discus F42
PGMWE3 Cycling Track, Men's Individual B Sprint
PGMWE4 Swimming Women's 200m Freestyle S14
As you can see, this single used stamp sold for £65+ on eBay. I wonder if there are any more out there in kiloware sacks?
This is the mint item referred to in the comments below. This sold for only £27, which seems odd, but it depends on who saw it at the time. Although the images look similar, the perforation runs across the legs just above the knees - compare the sky next to the Queen's head.
UPDATE 26 February:
Thanks to Anonymous commenter, we can show another misplacement which has been offered on eBay:
The overprinting - the design and caption - is shifted vertically resulting in the black overlaying the original gold caption, and the head and arm are above the level of the perforations.
We've been told about a sheet of Gold Medal stamps with duplicate printing of the black, but if the design and caption were printed in one pass of the press, the picture should be doubled as well. We're waiting for a picture!