Publishers have had to correct an error in one of JK Rowling's Harry Potter books, a fan website has revealed.

The hardback edition of Harry Potter and the Half-Blood Prince said Hermione Granger had scooped 11 top results in her Ordinary Wizarding Levels (OWLs). But readers spotted that in the preceding Harry Potter and the Order of the Phoenix, she had only taken 10 subjects.

Publishers Bloomsbury fixed the discrepancy in the next edition. The results almost exactly mirror the real-life GCSE grades achieved by Emma Watson, 16, who plays Hermione in the film adaptations.