MISTAKES
In the first Harry Potter, Harry buys One Thousand Magical Herbs and Fungi but near the end it says harry looked up dittany in One Hundred Magical Herbs and Fungi.
In Harry's letter from Hogwarts it says he could bring a cat, an owl, or a toad, yet Ron still brings his pet rat, Scabbers.
Chapter 3 says that Harry's 11th birthday was on a Tuesday, but according to JK Rowling, Harry was born in 1980 which would make Harry's 11th birthday July 31, 1991, which was a Wednesday.
Professor Dumbledore says that Nicolas Flamel destroyed the sorcerer's stone, and has just enough Elixar to set his affairs straight and then he and his wife would die. But earlier in the book, it says that the Elixar of Life makes the drinker imortal. So if Flamel drank it than he would never die.
Harry sees all his 'descendants' in the mirror. JKR means 'ancestors'.
In the first book, Petunia Dursley says that Lily would "...come home every vacation with her pockets full of frog spawn, turning teacups into rats...," but according to Magical rules, this is illegal. If she did this in her muggle home, she would most likely be expelled as Harry almost was.
When Hagrid is taking Harry down Diagon Alley for the first time, a woman says "17 sickles an ounce for dragon liver, they're crazy!" but Hagrid told Harry that 17 sickles makes a galleon. It would be like saying 100 pennies, instead of 1 dollar.
In chapter 7, "The Sorting Hat" they say after Harry's turn at the sorting hat that there are three more people to be sorted but there are really four. Here are the four people.... Dean Thomas, Lisa Turpin, Ron Weasley, and Blaise Zabini.
JKR wrote that Ron was the knight in the giant chess set! However, he later said that he had to 'move ahead one' so the Queen could take him, opening the path for Ron and Hermione to check-mate the king. If he was a knight, he could only move 1) ahead one, sideways two, or 2) ahead two, sideways one.
Harry's equipment list says 1 wand twice, once at top, once at bottom.
In Chapter 16 Ron says, "Well, Harry, you take the place of that bishop, and Hermione, you go next to him instead of that castle." But in chess, bishops and castles aren't side by side - the knight is placed in between them.
When the students are learning the "Wingardium Leviosa" spell Flitwick says "Never forget zard Barufio who said 's' instead of 'f' and ended up on the floor with a buffulo on his chest." There is no f in Wingardium Leviosa. It should have been f instead of s.
When Harry goes to the zoo and looks at the snake it says that the snake winked at Harry, but snakes have no eyelids so the snake couldn't wink at him.
Unless JKR made a mistake, Sirius Black had to have been in Slytherin. In Book One, Hagrid said, "There's not a single witch or wizard who went bad who wasn't in Slytherin". When he said that, nobody knew that Sirius Black was innocent. He was still thought to be a dark wizard. So if what Hagrid said was true, then Sirius Black was in Slytherin, unless Hagrid knew he was innocent.
When you go to Diagon Alley, you use your wand to get through the brick wall. If that is so, how do Muggle-Born students get in to the shops if their parents are Muggles and don't have wands?
I don't know if Malfoy buys his wand at a different store, but when he and harry meet in Madam Malkins, he says his mom is buying his wand. But the creepy old guy at the wand shop said "THE WAND CHOOSES THE WIZARD". Malfroy would have to be there to buy his wand.
It says that Harry drank the last of the potion that lets you go forward. Yet Dumbeldore still gets through the fire.
In book 2, Mr. Weasly got in trouble when Fred and George were spotted to fly his car. It's banned to "magic" muggle products. But as we all know, Sirius loved his flying auto-bike and he even lent it to Hagrid to let him bring baby Harry to Dumbledore in the beginning of book 1. Wondering why Sirius had not been in any sort of trouble for flying the muggle auto-bike??