That I am a huge Harry Potter fan is no secret (or why else would I have a category dedicated to it on my blog). I have read and re-read the books hundreds of times (no kidding), and upon re-reading the series a few weeks ago, I couldn't help notice a few mistakes and incongruences. Some of them have been since rectified by J.K. Rowling, and some are so blatant that I am still not sure whether I simply couldn't wrap my head around it completely, so let me know if I am sharing blunders here.
Let's start at the very beginning of the first book, shall we? In the book, it states that the story begins on a "dull, grey Tuesday"; however, we know that Harry's parents died on 31st October 1981, which, incidentally was a Saturday. As Harry comes to the Dursleys immediately after his parents perished, the story should start on a Sunday, not a Tuesday.
The wrong day, however, is not the only peculiarity about the beginning. At the beginning, the Dursleys have a normal day, watched by McGonagall as a cat and in the night from Tuesday to Wednesday Hagrid delivers Harry to his relatives. Now, in the evening, McGonagall and Dumbledore talk, the first saying wizards have been partying all day to celebrate Voldemort's demise, but that makes me wonder where little Harry was all day. Clearly, the tragic incidence of his parents dying happened the previous night and he is only delivered to the Dursleys the consecutive night, so where was he in between? Hagrid then also states "young Sirius Black" leant him the motorbike, but in Book 3, we learn Sirius was at the site of crime right after it happened and gave Harry to Hagrid before being arrested. Does that mean Hagrid needed more than a whole day to fly Harry from Godric's Hollow to Little Whinging? So many questions...
Let's walk over to Diagon Alley. Hagrid explains to Harry that "seventeen Sickles to a Galleon and twenty-nine Knuts to a Sickle." Just a few pages before, Harry overhears a witch saying, "Dragon liver, seventeen Sickles an ounce, they're mad." Obviously we don't know exactly how money works in the magic world (why is it sometimes so big, but can fit into a bag??), but wouldn't you say one Galleon instead of seventeen Sickles when it is seventeen Sickles to a Galleon? I never say, "Oh dear, 300 cents for an ice cream, golly me!" This mistake, however, was rectified in later editions. It now reads "sixteen Sickles".
Next up, another mistake that clearly should have served as handy exposition, but retrospectively is simply stupid. In the chapter The Journey From Platform Nine And Three-Quarters, Harry is desperate to find the right platform (thanks for being useless, Hagrid) and he overhears the Weasleys talking, pricking his ears when they mention "muggles". Molly Weasley, however, then asks, "Now, what's the platform number?", which is simply ridiculous. As far as we all know, in the Harry Potter series, the train ALWAYS leaves and has always left from this platform and Molly has bid five children farewell to Hogwarts so far, so her not knowing the platform by heart by now either means she's the greatest dimwit of all times (which she clearly isn't), or it was a back-handed method for some exposition.
We're staying with the Weasleys for a moment. We all know they're terribly poor and it makes sense that many of Ron's things are second-hand and he states in the same chapter that, "I've got Bill's old robes, Charlie's old wand and Percy's old rat." OK, Bill's old robes make sense, he doesn't need them anymore. Percy being kind of a dick giving up his old pet the minute he gets a new one also makes sort of sense but CHARLIE'S WAND? Remember, "The wand chooses the wizard, Mr. Potter?" Well, that's this statement down the drain. Why would Ron have Charlie's old wand? Wouldn't you only get a new wand when the old one broke? How is Ron expected to work properly at school when he doesn't even have a wand that chose him? No wonder he's scared of not living up to his brothers' standards, how could he...seriously. Also, a wand, as we learn later, costs 7 Galleons, so not really a fortune. We get the Weasleys are poor, but at least get your son a proper wand.
And another little thing about the Weasleys. When Ron and Harry finally arrive at Hogwarts, they have to wait in a little room for the sorting ceremony and everyone is nervous. Ron has no clue what's upon them, and I really wonder why. Sure, Fred and George may have told him he has to battle a troll, but in his ENTIRE family consisting of brothers and parents, has really no one told him what the sorting ceremony would be? That's kind of a dick move on your child. And fans who now rush forward, saying it may be a Hogwarts tradition to let eleven-year old kids wait in panic and agony, think about the scenario in Book 7, where Harry tells his son Albus he can make a choice in which house he'll be sorted (or not, poor Albus). So Harry told his son beforehand and this theory goes down the toilet.
Let's forward a little bit to the chapter Nicolas Flamel. In it, it states that "while copying down different ways of treating werewolf bites, Harry and Ron were still discussing what they'd do with a Philosopher's Stone (...)". Having recently read Book 6 again, I clearly recall Madam Pomfrey saying about Bill's wounds, "No charm will work on these. I've tried everything I know, but there is no cure for werewolf bites." Bill wasn't even been bitten by a proper werewolf, but his scars are cursed and will never fully heal, but in Book 1, it sounds as if there is not only ONE way of treating them, but various. A little inconsistent...
Lastly, I have noticed a little inconsistency that is addressed in the last chapter The Man With Two Faces. Voldemort taunts Harry with the details of how his parents died, saying "I killed your father first and he put up a courageous fight". However, in Book 7 we relive the scene and it is stated that James Potter comes running out, wand-less, and is killed quickly without even having time to put up a fight. And how would he without a wand?
Did I miss any obvious mistakes you spotted? Let me know :-) Also, stay tuned for mistakes of the next books, soon uploaded here.