The famous “11 O.W.L.s.” error

Where books are produced, mistakes are inevitable. Almost every newly published book contains one or more typographical errors, misplaced punctuation marks, spelling mistakes, or layout issues. When a book proves successful and goes through multiple reprints, such errors are usually corrected. As a result, they are often confined to the earliest printing. For collectors, these mistakes can serve as useful identifiers, helping to distinguish first printings from later editions. In that sense, an error can actually aid recognition.

A well-known example is the misspelling of the word “Philosopher’s” on the rear cover of the first printing of Harry Potter and the Philosopher’s Stone, where the final “o” is missing, resulting in “Philospher’s”. This error appears exclusively in the first UK printing. Because the print run was limited to just 500 hardback and 5,150 paperback copies, this issue is both rare and highly valuable.

Another notable example is found in the first hardback printing of the third title, Harry Potter and the Prisoner of Azkaban. This printing contains a layout error on the first page significant enough that the publisher halted the presses during production. Consequently, only a small number of copies exist, making this issue particularly scarce. In this case, the error directly contributed to the rarity of the first printing.

As the series grew in popularity, the size of each subsequent first print run increased dramatically. While only 500 hardback copies of the first book were printed in 1997, the initial print run of the final volume, Harry Potter and the Deathly Hallows, reached an astonishing 8 million copies just ten years later.

All of these first printings contained a considerable number of errors. Bibliographer Philip W. Errington identified nearly 100 errors across the first three titles alone, all of which were later corrected. It is therefore clear that such mistakes are only considered significant when they occur in relatively small print runs.

For reasons that are not entirely clear, certain errors found in books with very large print runs have nevertheless taken on a life of their own. One of the most widely cited examples is the error found on page 99 of the UK edition (page 100 in the US edition) of Harry Potter and the Half-Blood Prince. The text mistakenly reads “eleven ‘Outstanding’ O.W.L.s” instead of the correct “ten ‘Outstanding’ O.W.L.s.”

Copies containing this error are frequently offered for sale online, where it is often presented as a feature that enhances the book’s value. In reality, the opposite is true: rarity—not the mere presence of an error—is what drives collectibility.

The figures speak for themselves. The first UK edition had a print run of 6.3 million copies, while the US edition totaled 10.8 million copies. The so-called “11 OWLs” error can therefore be found in well over 17 million books....

Within a print run of this scale, the presence of a textual error does not confer scarcity, distinction, or added value. It is simply a printing mistake—one of millions.
Distinguishing between meaningful points of issue and widely repeated production errors requires careful bibliographical knowledge—an essential foundation for any serious collector.