I know he wasn't a prefect, but maybe once he was with Lily, calmed don't from jinxing everything and maybe he earned the right to be head boy, I'd hate to think that it was a writing error

Nat x

Suppose its another accidental mistake of j.k's. :|

I think the explanation given earlier in this forum is just covering Jo's messes... it would be seriously unfair to a 7th year prefect to give the headboy badge to someone who has not had the responsibility of being a prefect for the past three years and has proven themselves worthy...
Can you imagine if the headgirl badge had been given to Parvati Patil instead of Hermione (hypothetically of course) because Parvati had suddenly shown that she was headgirl material? That just doesn't make sense...

Actually, James was made the same rank as a prefect when he became Quidditch captain. Thus, making him eligible to be Head Boy. Make any sense?

He got to have the same privileges as the prefects when he became Quidditch Captain, like using the Prefects bathroom, but I don't think you can say he had the same "rank" as the prefects. Quidditch Captain can't give out punishments like prefects can, and they aren't expected to look after people in their house.

I really think, based on the canonical literature, that Rowling just made a slight error. She said James was Head Boy in the first book, but then wanted to make Harry not becoming prefect more acceptable in book 5 by saying James never did either. You can add after the mistake is found that you don't have to be a Prefect to be Head Boy/Girl, but that doesn't make much sense and seems to be just covering mistakes.

A prefect and a headboy are completely different things. A head boy is a higher rank than a prefect. If you remember, Percy weasley was both.