User blog comment:Morganaforever/Morgana's reasons/@comment-4135736-20120203092458/@comment-4135736-20120206125809

I think Merlin's biggest mistake has to be the fact that he turned his back on Morgana when he could have helped her himself (even possibly revealing his magic to her), this was much worse than the other mistakes he made like poisoning Morgana, forcing Kilgharrah to heal her etc. because that was the reason why Morgana became evil. This has to be the mistake the producers were talking about because this is the only mistake which was never mentioned in the series and which Merlin never realised himself (he currently thinks that he should not have told Morgana that she had magic but it is actually the opposite). When Morgana learns that he has magic, she may resent Merlin even more for not telling her and maybe not helping her out in the first place. That would be enough to make him realise that he could have helped her.