General Mistakes

• In Sorcerer's Stone, US paperback edition, page 96, it says that Percy had a silver prefect badge. However, in Order of the Phoenix, US hardcover edition, page 161, it says that the prefects' badges are scarlet and gold.
  
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• In Prisoner of Azkaban, page 262, Harry sends a Patronus at Malfoy and friends (who were dressed as dementors) during the Gryffindor vs. Ravenclaw Quidditch game where Lee Jordan is commenting.
  
  Then in Order of the Phoenix, page 342, Susan Bones asks Harry "So -- is it really true? You make a stag Patronus?" Harry says, "Yes." Lee responds "Blimey, Harry!...I never knew that!"
  
  But he would have seen it at the Quidditch game two years before.
  
  Possible Explanation: Harry's Patronus at the time of the Quidditch match may not yet have been developed enough to take the form of a stag.
  
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• In the American version of Prisoner Of Azkaban, page 429, it states that "The exam results came out on the last day of term." On page 430, it continues: "Percy had got his top-grade N.E.W.T.s; Fred and George had scraped a handful of O.W.L.s each."
  
  But in the American version of Half-Blood Prince, pages 79 and 99, Dumbledore says that the O.W.L. results are expected some time later today-- "today" meaning sometime during the summer holidays, and not while the students are still at Hogwarts.
  
  
• Dumbledore was Transfiguration teacher before he was Headmaster, and Remus Lupin was born in/around 1960 (in OOTP Snape is 35 or 36, so we can make an estimate on Remus' age in POA from that).
  
  Now: On page 352 of the hardback version of POA, in the chapter entitled "Moony, Wormtail, Padfoot, and Prongs," Lupin says, "I was a very small boy when I received the [werewolf] bite," and on the next page says "It seemed impossible that I would be able to come to Hogwarts... But then Dumbledore became Headmaster...", and of course Dumbledore allowed Remus to attend Hogwarts.
  
  Now skip ahead to OOTP. On page 321, in the chapter entitled "The Hogwarts High Inquisitor," McGonagall says she has been teaching at Hogwarts for "thirty-nine years this December." Assuming that McGonagall has always taught Transfiguration (never any other subject) at Hogwarts, and also assuming that at this point Remus would also be 35 or 36 (based on Snape's age), then Dumbledore would have been Headmaster for at least three years before Remus was born. So why did Remus say he wouldn't have been able to attend Hogwarts if Dumbledore hadn't become Headmaster after Remus was bitten?