Book One – The Sorcerer’s / Philosopher’s Stone

Sorting by Numbers

In chapter seven of the Sorcerer’s Stone, at the sorting, JKR writes that 'there are only three people left to be sorted'. However, Dean Thomas, Lisa Turpin, Ron Weasley, and Blaise Zabini were all left, meaning really she should have written four. This mistake has been apparent in some versions and non-existent in others.

Chess Lessons

In chapter sixteen of the Sorcerer’s Stone, Harry, Ron and Hermione take the place of the chess set pieces. Ron takes the place of a knight but when it comes to him sacrificing himself to the queen he only takes a pace forward. According to the rules of chess, moving a knight only one pace forward is an illegal move.

Nearly Headless Nick is Nearly 500

In chapter eleven of the Sorcerer’s Stone, it is almost Nearly Headless Nick's 400th death day (years since he died). But in Chamber of Secrets, he celebrates his 500th death-day.

How many Herbs and Fungi?

In certain versions of the Sorcerer’s Stone, One Thousand Magical Herbs and Fungi is at one point called One Hundred Magical Herbs and Fungi as an error!

Book Two – Chamber of Secrets

Confused Clock

In chapter three of the Chamber of Secrets, the Weasley's clock shows phrases such as ‘Feed the Chickens’. In Goblet of Fire, it only states the locations of family members. We may be able to conclude that the Weasleys have two clocks. In Chamber of Secrets, the clock is described as hanging on the wall and in Goblet of Fire, it is the 'old grandfather clock'. Grandfather clocks aren't known to hang on walls.

Myrtle lives where?

In chapter eight of the Chamber of Secrets at Nearly Headless Nick’s death-day party, Hermione states that Moaning Myrtle haunts the girls' bathroom on the first floor. However, it is later proven that the bathroom is on the second floor. Also, it states at two different points that Myrtle haunts the U-bend of the toilet (Chamber of Secrets), and then the S-bend of the toilet (Goblet of Fire). Make up your mind, J.K.!
Thanks to Mira Miles

Perry Weasley?

In chapter nine of Chamber of Secrets, after PERCY says "Get--away--from--there--" his name is misspelled as "Perry".

Slytherin, Ancestors or Descendents?

In chapter eighteen of the Chamber of Secrets, Dumbledore explains to Harry why he can speak Parseltongue. In this explanation, he refers to Lord Voldemort as "the last remaining ancestor of Salazar Slytherin." He quite obviously means descendant, not ancestor, unless the Slytherin family reproduces differently from the rest of humanity.

Book Three – Prisoner of Azkaban

Money Mishap

In chapter three of the Prisoner of Azkaban, Stan tells Harry that the price to get to London was eleven sickles. Then, it said Harry shoved some gold into Stan's hands. Sickles, in fact, are silver.

Marcus Flint, 8th year student?

In chapter eleven of the Sorcerer’s Stone, the book tells us that Marcus Flint is the Slytherin Captain in his sixth year. That means Harry's second year is Marcus Flints seventh and final year at Hogwarts. In chapter fifteen of the Prisoner of Azkaban on page 306, fourth line, Wood and Flint shake hands. This year would be Flint's eighth year. You are only supposed to go to Hogwarts for seven! An arguable error indeed, J.K. Rowling has since clarified that Flint has held back a year, which makes sense considering Flint's character.

Snape or Snap?

In chapter fourteen of the Prisoner of Azkaban, on the second line from the bottom of that page you will see Snape's name spelled as "Snap"! Found in first American Edition, Hardcover.

The Keeping Seeker

In chapter fifteen of Prisoner of Azkaban, 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.
-Thanks, MuggleNet

Headmaster Albus Dumblefore?

In chapter twenty-two of the Prisoner of Azkaban, close to the end when Harry and Dumbledore are talking in Lupin's office after he left, J.K. spelled Dumbledore's name as Dumblefore. This is found only in American First Edition books.

Book Four – Goblet of Fire

Something Broke

In chapter seven of the Sorcerer’s Stone, Professor McGonagall brings out a four-legged chair with the sorting hat (to the sorting ceremony). In chapter twelve of the Goblet of Fire, she brings out a three-legged chair with the sorting hat. What happened to one of the legs?
-Thanks, MuggleNet

Dark Mark, Where?

In chapter twenty-five of the Goblet of Fire, when Harry gets stuck in the stairs and Snape and Moody come along, Moody makes some statement. I forgot what it was, but it was something pertaining to the Dark Mark on Snape's arm. Snape suddenly grabs his right arm; therefore the mark should be on that arm. But later, he shows Fudge the mark on his left arm...

Priori Backwards

In chapter thirty-four of the Goblet of Fire, when Priori Incantatem was preformed, the spells that Voldermort's wand cast out are all the people he has killed, from his most recent to his first. However James Potter had appeared before Lily, stating that Lily was killed first, which we know for a fact is not true. This mistake has been omitted on later versions released.

Moody...No! It's Crouch!

In chapter thirty-five of the Goblet of Fire, the spoken line is: "The Imperius Curse," ‘Moody’ said. "I was under my father's control. I was forced to wear an Invisibility cloak day and night." Oops! Why is Moody saying this? It should be Crouch saying it.

Charlie works in "Rumaina"

In the Goblet of Fire, Bill Weasley says that he has not been to Hogwarts in five years. Since Harry is in his fourth year that would conclude that Bill had left the school the year before Harry joined Hogwarts. This cannot be true because Bill is the oldest child of the Weasley family. Charlie (the second oldest) is younger than Bill, but he was not at school when Harry was. Charlie probably left the school at least three years before Harry came. We can determine this because Fred and George were talking about how they had not won a single game since Charlie left (Fred and George said this in Harry's first year). They probably would not have been talking as though Charlie had left the year before. This was pointed out to J.K Rowling, she said that Bill visited the school for one of Charlie's Quidditch matches.
-Thanks to cfsdimples.

Who’s Talking to Who?

According to the paperback edition of Goblet of Fire, "Crouch is talking to Dumbledore" in one section, but in the hardcover edition, "Fudge is talking to Dumbledore."
-Thanks to gazoo91.