If you own a copy of the book Harry Potter and the Philosopher’s Stone, there’s a chance it could be worth tens of thousands of dollars. How do you know if yours is one of the 500 hardbacks with the rare mistake? Go to page 53, and look at the list of school supplies recommended for incoming Hogwarts students. If “1 wand” is on there twice, you could be sitting on a small fortune (that is, if your copy is still in good condition). As Time and The Independent note, one copy might bring more than $30,000 at an auction this fall.

As an official at Bonhams Fine Books and Manuscripts, who is hosting the auction, explained to The Independent, “Like most enduring books aimed at younger readers, the Harry Potter books also have wide appeal to adults and there is a strong market among collectors for first editions.”

Here’s a look at the passage a Twitter user discovered back in 2014: