In the first editions of Half-Blood Prince
, an error discovered in "An Excess of Phlegm" stated that Hermione received one Exceeds Expectations and ten Outstanding grades in her OWLs. However, she only took ten subjects. Our Potter Encyclopedia
had staggered a guess that she took the "Muggle Studies exam and passed it with top marks despite dropping the subject at the end of her third year."
reports that this was indeed a genuine error and has been rectified in recent editions of Book 6; it now reads she earned nine Os and one EE. Phew!