On page 101 of the earliest printings, and in the original audiobook editions of Harry Potter and the Half-Blood Prince, Ron mentions that Hermione got ten Outstandings and one Exceeds Expectations for Defence Against the Dark Arts giving her a total of eleven O.W.L.s. However, as she dropped both Divination and Muggle Studies in her third year, she could not possibly have got eleven O.W.L.s as she was only doing ten subjects - Transfiguration, Charms, Herbology, Potions, Defence Against the Dark Arts, History of Magic, Astronomy, Care of Magical Creatures, Arithmancy and Ancient Runes. This error was corrected in all later releases, in which Ron instead says nine Outstandings and one Exceeds Expectations.
Harry's sixteenth birthday happens on July 31st, 1996. Looking at any full moon calendar, in that year full moon was on July 30th. Remus Lupin was recovering from the after effects of his werewolf transformation and it is therefore difficult for him to go to birthday parties. It is said, though, that he was looking "gaunt and grim," so he might have transformed the night before. At Christmas, however, it is stated that Remus Lupin comes over to the Burrow and even uses Bill's bedroom. December 24, 1996 is the exact date of a full moon and, since Lupin was already living with the werewolves, there was little chance he could get the seven days doses of Wolfsbane potion required to make him harmless. Therefore, on that night he would have been a dangerous transformed werewolf.
In the chapter titled "The Secret Riddle," It says, "... Dumbledore entered the room ... It was a small bare room with nothing in it except an old wardrobe and an iron bedstead." A couple paragraphs later, "Dumbledore drew up the hard wooden chair beside Riddle, so that the pair of them looked rather like a hospital patient and visitor." This may have been a pun, as with Dumbledore especially, the term "draw up a chair" is used literally several times when Dumbledore magically conjures a chair by drawing it. This may have been a covert way of implying this, especially considering Riddle may have missed this action by Dumbledore.
On page 342 of the American version, there is a punctuation error when Molly Weasley says "Have a little purkey, or some tooding...I mean-" to Rufus Scrimgeour. There are no quotation marks in front of the have in the statement. This mistake is corrected in later printings.
In Snape Victorious Snape deducts 50 points from Gryffindor so that they're in the negative, but at the end of book 5 Snape can't deduct points as Gryffindor doesn't have any. An explanation of this is that on the latter occasion, Snape says, 'We'll just have to'... before Professor McGonagall appears and adds more points. It is possible that Snape was about to put the Gryffindor points into negative figures.
At the start of term feast, Dumbledore tells Quidditch hopefuls to put their names down for selection. For the Gryfindor House at least, there had never been trials before. Katie Bell also warns Harry against just "picking the old faces" but this had automatically been the case for the 5 years preceeding Harry's 6th year. It is likely Wood was happy with the team for those five years, and there was no reason to hold a trial with no spots becoming vacant up until he left. This may also be a sign of Harry simply bowing to pressure, as he could have chosen to allow those that were already on the team to continue playing, but instead chose to take Katie's advice.
