Ok... i can give you answer for some of this so called "errors"...
Fluffy bear wrote:Whilst shopping in Diagon Alley, Harry overheads a couple of witches compalining about the price of Dragons liver, "Seventeen sickles an ounce for Dragon Liver. They're mad." But 17 Sickles are equal to 1 Galleon, so she should say 1 Galleon an ounce. What she said would be the same as saying '100 pence an ounce' instead of a pound.
I don't know where do you live, but I'm from Portugal, and we use the Euro. I often say 100 cents instead of 1 Euro, cause I understand the value of the money better this way...
Fluffy Bear wrote:At the end of the second book Dumbledore tells Harry that Lord Voldemort is the last remaining ancestor of Salazar Slytherin. But it should say that Lord Voldemort is the last remaining descendant of Salazar Slytherin.
It was corrected in later versions.
Fluffy Bear wrote:Harry uses the Marauders Map throughout the book, but it was taken from him in his fourth year. When and how did he get it back?
It was taken by Moody, and DD could have given it to Harry once he knew
the true identity of Crouch Jr.
Fluffy Bear wrote:Harry see Thestrals for the first time in this book(because he has 'seen death'), but Harry saw his mother die when he was a baby. And why didn't he see the Thestrals at the end of GoF just after he'd seen Cedric die? (JK Rowling's response: You can't see the Thestrals until the death "sinks in.")
Harry wasn't fully aware of his mother's death. He didn't really see is mother die, but only realized that much much later.
I believe the others are real errors, but maybe the one on Dennis Creevy is the only one that might be considered a real error: can just be corrected printing a new version, without changing anything else on the book.