There was a few occasions whereby I did amend the failing grade like the poster Mr gerago did. It wasn't too difficult a task if one had the trust, credibility and stature that the company and regulatory body respect. I had no compunction whatsoever in undoing my mistakes when it was fully justified to do so.
Well, well, well...so your stature was such that the company and the CAA would acquiesce to your request that the failing grade you awarded be amended. Good on you and I am sure you think that makes you feel good. This kind of sound rich but it is better late than never in correcting your mistake.
However, bear in mind that flying especially flying in command has a lot to do with confidence. A pilot's confidence is often badly damaged, if not destroyed by failing grades especially when done maliciously. I have seen friends whose confidence plummeted drastically on account of arsey checkers like you; hence my advice is that checkers bear that in mind before they embark on their jehovah moments.