1. The number "7" is a very important and very magical number in the Harry Potter universe. We all know that there are seven Horcruxes...

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...but there were also seven total Harry Potters in The Deathly Hallows Part I...

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...seven locks on the chest that was holding Mad-Eye Moody...

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...seven players on a Quidditch team...

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...seven snake heads on the door to the Chamber of Secrets...

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...the number seven on Harry's Quidditch Jersey...

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...and Ginny is the seventh Weasley child, the first girl to be born to a Weasley in seven generations. 

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It might not be a coincidence that there are seven Harry Potter books. 

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2. Before Harry goes to Hogwarts, he talks to a snake at the zoo. Out front you can see children wearing a deep green -- the color of Slytherin

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via Angry_Foamy



3. Ron's robes are visibly more faded and worn than anyone else's, because they are hand-me-downs from his brothers

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via Knottsville



4. In Deathly Hallows Part 2, Snape sneakily deflects spells onto his Death Eater "friends" while fighting McGonagall, going so far as to bounce around and grab their wands before escaping -- even as a "bad guy," Snape is still helping Hogwarts

via bobcobble



5. In Chamber of Secrets, Harry kills the Basilisk...

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...and it falls onto the floor...

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Years later, in Deathly Hallows Part II, the decayed corpse of the Basilisk can be seen rotting away

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(yeah, it's part of the plot, but it's still a neat background detail that they don't over-emphasize)

via bloomin__onions



6. The kids eat parody cereals like "Cheeri-Owls"

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via minisaladfresh



7. The end of Professor Slughorn's wand is shaped like slug "eyes"

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via TheTurnipKnight



8. In Chamber of Secrets, after Dobby is freed you can hear Lucious Malfoy start to utter "Avada Kedavra" -- the killing curse -- before he is stopped

via Evolations



9. In Prisoner of Azkaban, you can see Buckbeak poop on-screen



10. The intro with the WB logo gets darker with every entry, just like the series itself

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via Greendead



11. In the first movie, you can see that The Leaky Cauldron's sign starts out black and fades into view as Hagrid and Harry approach -- the sign is hiding from muggles

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via Cronamania



12. Gilderoy Lockhart is secretly bald -- you can see a spare wig on his desk

Brilliant Harry Potter Movie Details You Might Have Missed

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via firejetfire



13. You may have seen this one floating around -- during the credits of The Prisoner of Azkaban, you can see what looks like two people messing around on the Maurader's Map

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14. In Prisoner of Azkaban, a wizard can be seen reading "A Brief History of Time" by Stephen Hawking

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via balint-uni



15. The last line of the Harry Potter books reads: "The scar had not pained Harry for 19 years. All was well." Sure enough, in the last shot of the last movie, Harry's scar has almost faded completely

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via minisaladfresh