So this sheet is something of a surprise. Bought at a Post Office branch, it shows an upward shift of the digital overprint by approximately 6mm.
This results in the athlete image overlaying the perforations, and the caption being printed over the preprinted gold text.
Only one miniature sheet has so far been reported.
UPDATE: Well, that didn't last long. Our anonymous contributor in the comment below has forwarded an image of a complete A4 sheet, with an even greater shift - about 9.5mm. And being a complete sheet, it shows that the printer was in Preston. Here's the lower left corner: And from this even more than the other, it is evident that the Team GB logo is printed at the same time as the athlete image, by the digital printer.
We understand that the regional printers were selected after several trials and tests, over a 6-month period. The test sheets (call them proofs if you like but they weren't actually proofs of the finished GM stamps) were examined by Royal Mail before the six printers were actually selected.
To be approved, the proofs had to meet the exacting specifications laid down by Royal Mail (curves, densities, dot-gain etc etc.), although the results were not necessarily what the printer would have normally accepted. (As you will see when my comparison scans are shown, the results vary considerably so I'm not certain that all 6 printers kept to the same 'exacting specifications'.)
The Taekwondo medal was won around 10.35pm, the artwork reached the printers at around 11:45pm, the plates were on press about 1am, with guillotining around 3am. If these misprinted sheets were the result of the set-up (or make-ready) process, and were put into the pile of 'good' sheets in error, then that is when it happened.
The Postmaster at Flint confirmed that he started selling the stamps at midday on the day of release, and within a couple of minutes had received a call from royal mail advising him to stop selling the stamps until he had checked them all. Apparently he had ‘several’ in his safe to return to Royal Mail.