Philosapher's Stone
We know that Harry's birthday is on July 31 because that is the same day as the break-in at Gringotts. From the second book we know Nearly Headless Nick's 500th Death Day Party was on 10/31/1992. Doing the math, we know that Harry's 11th Birthday was 07/31/1991. In The Sorcerer's Stone, Harry says his birthday is on a Tuesday, the day after Dudley's favorite TV show (The Great Humberto). The mistake is that July 31, 1991 fell on a Wednesday, not a Tuesday like the book said.
Arguable: When Hagrid brought Harry to Diagon Alley for the first time, Harry heard a woman say "Seventeen sickles an ounce for Dragon Liver. They're mad." But Hagrid told Harry that there were seventeen sickles in a galleon. So that would be like saying "100 pennies" instead of a dollar.
In Harry's letter from Hogwarts it says he could bring a cat, an owl, or a toad, but Ron brings his rat, Scabbers.
It says in Book 1 that Marcus Flint is in his 6th Year (American book). He is still at Hogwarts in Book 3. This would mean he was in his 8th year and students only have seven years at Hogwarts. Explanation: JK said in an online interview "He had to do a year again! :-)."
Arguable: In the first book it said that by drinking the Elixir of Life it would make the drinker immortal, but later in the story Dumbledore says to Harry that the Flamel's had enough Elixir stored in them to set their affairs straight. But, if drinking the Elixir of Life makes you immortal then if you had some, you would never die, ever. You can't just be immortal as long as you have a certain item, then that would mean you could still die. You would be still mortal like Achilles. Immortality means you can never die.
The Chamber of Secrets
Dobby seems to be apparating and disapparating inside the grounds of Hogwarts. He also seemed to apparate and disapparate while Harry was in the hospital wing recovering from his arm accident. But in Harry Potter and the Prisoner of Azkaban, Hermione states numerous times that it is impossible to apparate and disapparate inside the grounds of Hogwarts. How can this be? -Thanks to Nick. Possible Explanation: House elves have their own magical powers and can disappear at will in a different way to apparating.
There is a typo on page 157. It says '"Get-- away--from-- there," Perry said striding towards them.' It's supposed to say Percy not Perry.
After Fred, George, Ron and Harry arrive at The Burrow after flying the car, Mrs. Weasley shouts at Fred first; "You will not," snapped Mrs Weasley. Then she says "And you two," glaring at Ron and Fred. It should say George as she has already spoken to Fred
At Nearly Headless Nick's Deathday party, Hermione says that Myrtle haunts the bathroom on the first floor. However, she should say on the second floor. Just before they find the writing on the wall, it says "Harry hurtled around the whole of the second floor" and when Hermione is talking to Myrtle, she says "because a cat was attacked just outside your front door on Halloween." Her bathroom is on the second floor, not on the first
Prisoner of Azkaban
In Chapter 19, The Servant of Lord Voldemort, page 370, it states that Voldemort had been hiding for 15 years. If his power was lost only twelve years ago, why was he hiding that extra three years?
In chapter 3, The Knight Bus, the driver Stan told Harry that the price to get to London was 11 sickles. But in the book it said Harry shoved some GOLD into Stan's hands. Sickles are SILVER, not gold.
In chapter 15, The Quidditch Final, it said Madam Hooch's whistle rang out as she soared over to Montague and began shouting at him. A minute later, Katie had put another penalty past the Slytherin SEEKER. It should say the Slytherin Keeper.
Page 428, chapter 22: it took a moment for Harry to realize what a person called "Dumblefore" had just said. Note: This was fixed in later versions.
