In the first edition of Harry Potter and the Chamber of Secrets, there is a passage:
"You can speak Parseltongue, Harry," said Dumbledore calmly, "because Lord Voldemort - who is the last remaining ancestor of Salazar Slytherin - can speak Parseltongue."
Here, there is an obvious mistake as Voldemort is the last remaining descendant of Salazar Slytherin. This error is fixed in a later edition. On which edition is this fix first applied?
On October 16th, 2000, when J.K. Rowling was asked a direct question about whether the Dark Lord was Salazar Slytherin’s ancestor or descendant, she says it was an error that has been corrected in subsequent editions.
Question: Harry Potter for grownups again! Is Voldemort the last remaining ancestor of Slytherin, or the last remaining descendent of Slytherin?
J.K. Rowling responds: Ah, you spotted the deliberate error. Yes, it should read "descendent." That's been changed in subsequent editions. (Keep hold of the "ancestor" one, maybe it'll be valuable one day!)
- J.K. Rowling interview on Scholastic.com (October 16, 2000)
Presuming the information she gave there is correct, that means the error was first corrected in an edition published before October 16th 2000, the date of that interview.
