Corrected entry: In the first potions class, Slughorn shows them a few potions and says that these are some of what the students should be able to prepare after completing their N.E.W.Ts. But later on in the book, Ron asks Harry to check his textbook to see if they can prepare a stock of it for themselves. Harry says it's very complicated and that it takes six months to stew. If that were so, how did Slughorn prepare it for their first class? He barely had even 2 months since Dumbledore hired him.
Corrected entry: When Harry and Dumbledore are in the second chamber in the cave it says, "The greenish glow and the light from the two wands." There should be only be one wand lit which is Dumbledore's, because it never says Harry lights his wand, and it is illegal for Harry to use magic outside of Hogwarts because he is not "of age".
Corrected entry: Malfoy used the Imperius curse on Madam Rosmerta to have her deliver the cursed necklace to a Hogwarts student. If Hogsmead is "outside of school" and Malfoy was underaged at the time, shouldn't the Ministry investigate the use of an Unforgivable Curse so close to Hogwarts? And if so, wouldn't they have found out that Rosmerta was cursed?
Corrected entry: It states in the Half-Blood Prince that the ministry can only detect magic, not its perpetrator. Therefore, when Dumbledore visits Harry at the beginning of the book and performs magic, shouldn't the ministry have detected it, knowing that Harry was an underage wizard? (There should have been no magic being performed at the house at all).
Corrected entry: In the US version, page 485, Slughorn tells Harry that he has "a house-elf taste every bottle after what happened to your poor friend Rupert". Slughorn should have said "poor friend Ron", not Rupert. One assumes this is a reference to Rupert Grint, who plays the character Ron in the movies.
Corrected entry: In the American edition, page 188, Slughorn says, "One tiny bottle of felix felicis is enough for twelve hours' luck. From dusk til' dawn, you will be lucky in everything you attempt." But then on page 476, Harry says, "I don't reckon I'll need all of it, not twenty-four hours' worth."
Corrected entry: In the part of the book where Harry holds Quidditch trials the book says that there are first years holding the old school brooms, but in the first book it says that first years are not allowed to have brooms or be on the house Quidditch teams.
Corrected entry: It is repeatedly said through the books that no one can apparate or disapparate on Hogwarts' grounds. However, both Dobby and Kreacher apparate to Harry when he calls them. [It has been argued that house-elves have magical powers of their own that aren't affected by wizard law. So it's not entirely unplausible that Dobby and Kreacher can Apparate around the castle, even with the anti-Apparation charm in effect.]
Corrected entry: In this book, during the end, Snape says that he is the Half-Blood Prince. Earlier, when Harry is at the Weasley's for Christmas, he wonders if his dad was the Prince. He looks in the book, and it says that the book was from fifty years ago. He is depressed by the fact that his dad wasn't at Hogwarts fifty years ago. In the fifth book, when Harry is in Snape's pensieve, he sees his dad and Snape finish their O.W.L.s. This would mean that they were in the same year at Hogwarts.
Corrected entry: On page 425 of the US version, it says '"Nice commentary last match." said Ron to Luna as she took back the green onion, the toadstool, and the cat litter.' But a couple of paragraphs later it says '"What is this, by the way?", he added, holding the onionlike object up to eye level.' Didn't she already take back everything a few lines ago?
Corrected entry: In chapter 23, "Horcruxes", at the end of Slughorn's memory Slughorn told Riddle not to mention to anybody that they have talked about horcruxes because Dumbledore is very strict on that point. But at the time the headmaster wasn't Dumbledore, when Riddle was a student the headmaster was Dippet.
Corrected entry: In the chapter "Horace Slughorn", in the house Harry sees 'something darkly red and gluttonous,' which is later described as dragon's blood. However in the fifth book, the chapter "Hagrid's Tale", Hagrid uses dragon meat on his wounds, but it says that the meat oozed 'greenish blood'.