Fred and George Weasley had the Marauder's Map before Harry did, so if Harry can see Peter Pettigrew in the map, then Fred and George seemingly ought to have noticed that Pettigrew was sleeping with Ron every night. They did, however, tell Harry that they found the map in their first year, and no longer need it as they "know it by heart", so it is possible that they have not used it since Ron (and Scabbers) arrived at Hogwarts, or that they believed that the map made a mistake, that conclusion having been drawn by Harry. It is also entirely possible that the twins simply had no interest in using the map to look at their brother sleeping.
The first chapter says that A History of Magic was written by Adalbert Waffling, rather than Bathilda Bagshot (rectified in later editions).
Given that Remus Lupin had a transformation sometime after Halloween but before the following weekend (the day of the Gryffindor-Hufflepuff match), the full moon would have had to be sometime in the first week of November, probably the 4th or 5th of November as he was absent from class on the Friday before the match. As the full moon occurs every 29-30 days, by that time frame the December and January full moons would also have occurred sometime around the 3rd-5th of those months. Therefore a full moon during the Christmas Eve to Boxing day period is not possible. Yet Dumbledore explains to Trelawney that Lupin's absence from the Christmas dinner is that he is not well again - implying either a transformation or post-transformation exhaustion, especially as Snape is mentioned as having brewed his potion for him. It is not specified, however, in the books how long a werewolf might suffer ill-effects from transformation, and how much variation there is between months. Therefore, he may simply be suffering pre-transformation effects and, as the Wolfsbane potion must be taken in the week preceding the full moon, then the next full moon may be as late as New Year's Eve. Notably, Snape says in the Shrieking Shack that Lupin had not taken his potion tonight, but it is explained he must drink the Wolfsbane Potion the week before transforming, not on the night itself, so missing the potion shouldn't have affected him within the space of a few hours.
Before Harry and Hermione go back in time, it mentions that Buckbeak is tied to a tree. But when they go back in time, Buckbeak is tied to a fence.
When Harry is buying his books at Flourish and Blotts, it mentions that he is being served by the manager. But when Harry was staring at the book 'Death Omens,' it says that he is being served by the assistant of the store, though it is possible that the manager was simply the assistant to the shop owner.
In June, Fred and George Weasley find out they have scraped "a few O.W.L.s apiece." However, in subsequent books, it is firmly established that O.W.L. results are sent home by owl and are not received until August. Yet it is possible that they snuck into an office and retrieve the information of their results or were assuming that they got low marks.
Mr. Weasley says that Ron and Harry have ended up in the Forbidden Forest twice. But only Harry has been in the forest twice. Ron went once to see Aragog with Harry in the 1992-1993 school year, but the other time Harry went was for detention in the 1991-1992 school year, and Ron was not there (he was in the film, but not in the book). This is most likely a mistake on Mr. Weasley's part rather than the author's.
Chapter sixteen (Professor Trelawney's Prediction) takes place in June 1994. In the beginning of that chapter, Harry Potter reads a note from Hagrid and says, "Buckbeak's appeal — it's set for the sixth." The night after the Hippogriff's appeal, Remus Lupin is forced into a werewolf form by the full moon, which means that this happened on the night of June 6th, 1994. The web page HP-Lexicon.org also backs this up. However, a brief look at lunar phase calendar sites such as 1994 and lunar phases in 1994 definitely shows June 9, 1994, as a new-moon night, which means that a full moon could not possibly rise three days earlier.
On the American version of the cover, the front art is the night that Harry and Hermione save Sirius and Buckbeak. However on the back side of the cover (Lupin appears as a werewolf), the moon is crescent when it should be a full moon.
The cover of the Scholastic edition depicts Harry and Hermione riding Buckbeak. Harry's legs are shown to be in front of Buckbeak's wings. However, when Harry was in Care of Magical Creatures and rode Buckbeak for the first time, Hagrid had told him to sit behind the wing joint (as he did in the movie). Although, it could be because Hermione is sitting behind him.
Sirius Black says that Voldemort has been in hiding for fifteen years. He has actually been in hiding for twelve years (almost thirteen at the time that he says it), about the same amount of time since Black was imprisoned. Voldemort went into hiding right after he was ripped from his body after failing to kill Harry. Of course, Black could just be rounding off to an approximated number (fifteen years), but this seems unlikely since he keeps talking about being in Azkaban for twelve years, being that the two time frames are almost exactly the same. Sirius also says to Pettigrew "you haven't been hiding from me for twelve years, you've been hiding from Voldemort's old supporters." Voldemort could, however, have been hiding to escape capture and operating secretly before killing Lily and James Potter.
In the last chapter on page 284, it says Dumblefore instead of Dumbledore.
On the back cover it says Harry blew up his horrible Aunt Petunia instead of Aunt Marge
Ernie Macmillan's last name is misspelled McMillan.
On page 379-380 of the American paperback edition, when exiting the tunnel that lead out of the Whomping Willow, it was Crookshanks, Lupin, Pettigrew, Ron, Snape, Sirius, Harry, and Hermione. Later on when Hermione used the Time-Turner (page 408), and she and Harry were watching the Whomping Willow to see themselves coming out the order changed to Lupin, Ron, Pettigrew, Hermione, the unconscious Snape, Harry, and Sirius. They could have simply missed Crookshanks exiting, though there is no explanation for the change in order of the visible humans.
Sybill Trelawney told Lavender Brown that the worst thing for her would come true on Friday, 16 October. However, 16 October, 1993 was a Saturday. This isn't a mistake, Trelawney rarely makes true prophecies, and is known to be a bit of a drinker; she may simply have been drunk and forgotten the date, or better yet, was making up the idea to merely scare Lavender. It is a mistake because they had lessons the day Lavender found she'd lost her rabbit.
When Harry gets back into the Gryffindor common room shortly before the Gryffindor-Ravenclaw match with his Firebolt given back to him after being tested for jinxes, the common room admires the broom. An unidentified Gryffindor tells Harry, "Ravenclaw'll have no chance, they're all on Cleansweep Sevens!" However, later in the book, Oliver Wood tells Harry that Cho Chang, the Ravenclaw Seeker, rides a Comet Two Sixty. It is possible, however, that the unidentified Gryffindor who told Harry this probably doesn't know much about the Ravenclaw Quidditch team or meant that most, not all, of the Ravenclaws ride Cleansweep Sevens.
On page 101 of the American hardcover edition, Sybill Trelawney's name is misspelled as 'Sibyll'.
In Chapter 15 (The Quidditch Final) when Katie Bell is taking her second penalty shot at the Slytherin vs. Gryffindor, it says she put it past the Slytherin Seeker. The Seeker in Quidditch is the one who chases the Snitch, which, in the case of Slytherin, would be Draco Malfoy.
