In Harry Potter and the Chamber of Secrets, a wig can be seen on the table of Professor Gilderoy Lockhart, probably suggesting that he is bald.
2. In Harry Potter and the Chamber of Secrets, “Sherbet Lemon” is the password used to enter Dumbledore’s office. Albus Dumbledore was particularly fond of these Muggle sweets and in Harry Potter and the Half-Blood Prince, there’s a bowl of them on his desk.
3. The opal necklace, which plays an important role in Harry Potter and the Half-Blood Prince, can briefly `be glimpsed in the second movie inside a display case in Borgin and Burkes’ shop in Knockturn Alley when Harry first enters it.
4. In Harry Potter and the Prisoner of Azkaban, Hermione can be seen tucking away her Time-Turner early on. Throughout the movie in fact, pretty much anytime someone comments on how she got there or when she isn't wearing her jumper or robe, the Time-Turner is visible around her neck.
5. Early in Harry Potter and the Prisoner of Azkaban, Harry sees a newspaper article with a photograph of the Weasley family in Egypt. Though it is never again mentioned in the movie, this is a very important plot point in the book. While on an inspection of Azkaban, the Minister of Magic had a copy of that paper, gave it to Sirius Black, who recognized that Peter Pettigrew was disguised as Scabbers. This is why he decided to break out of Azkaban, track down Scabbers at Hogwarts, and take his revenge by killing him.
In Harry Potter and the Goblet of Fire, in the scene with Dumbledore, Harry, and the Pensieve, there is a 3D model of what looks like Hallows Symbol which becomes an important symbol in Harry Potter and the Deathly Hallows. Though the final book was not yet written when this movie came out, J.K. Rowling had a lot of influence over the movie details and she added lots of objects and plots in the movies that were referenced later.
7. In Harry Potter and the Goblet of Fire, when the contestants enter the arena for the third task, the Beauxbatons girls in the audience can be seen dancing the Macarena.
8. During the breakfast scene of the Great Hall in Harry Potter and the Order of the Phoenix, students can be seen eating parodies of real-world cereal brands with the names such as Cheeri-Owls and Pixie-Puffs, with color schemes similar to those of Cheerios and Sugar Puffs boxes, respectively.
9. In Harry Potter and the Order of the Phoenix, James Phelps and Oliver Phelps, who play Fred and George in the franchise, make a cameo appearance in the picture of the original Order of the Phoenix as their deceased uncles, Fabian and Gideon Prewett.
10. In Harry Potter and the Order of the Phoenix, when Sirius joins Harry in the room with the Black family tree, to the lower left of the door is a Starbucks logo. The “siren” has been stylized to be in keeping with the decor of the room, but she is wearing a crown and holding both her fins aloft as in the Starbucks logo.
In Harry Potter and the Half-Blood Prince, the smoke coming out of the love potion changes color to that of whatever Hermione is describing. The final scent that comes out of the potion is orange, as is Ron's hair. That's why Hermione lies saying that she smells “spearmint toothpaste” because she obviously doesn’t want to say “Ron’s hair” out loud. Also, after Harry arrives at the Burrow, Ron pokes Hermione’s lip and says she has toothpaste on it.
12. In Harry Potter and the Half-Blood Prince, Dumbledore reaches for a Tom Riddle memory dated 1938. He tells Harry that this was the first time he met Tom, but to the left, there is a memory labeled Tom Riddle that is dated 1937. It is most likely someone else's memory relating to Riddle terrorizing two children in a cave at the seashore, which is described in detail in the book.
13. Arthur Bowen, the child actor that plays Harry's son Albus Severus Potter in the epilogue of the final movie, can be seen in Harry Potter and the Half-Blood Prince buying roasted chestnuts at a stand in the Diagon Alley.
14. In Harry Potter and the Deathly Hallows - Part 2, Professor Snape still helps the Order when he redirects McGonagall’s spells to knockout his fellow Death Eaters. He also takes their wands before he flies out the window to make sure they could no longer harm any students. In the books, there are no Death Eaters around him so it was a nice touch to add this detail to the movie.
15. In Harry Potter and the Deathly Hallows - Part 2, Lucius Malfoy’s Azkaban prisoner number tattoo can be seen on his neck for a brief moment. After Order of the Phoenix, he and other Death Eaters were arrested after they were caught in the Ministry. He only gets out when Voldemort started to take more power by Deathly Hallows.
In Harry Potter and the Deathly Hallows - Part 2, when all the professors leave the castle to cast the defensive shield, Professor Slughorn can be seen drinking a bottle of what looks like liquid luck.
17. In Harry Potter and the Philosopher’s Stone, the Dursleys and all their neighbors own the exact same make and model of car, in an effort to be as normal as possible and blend in with everyone around them.
18. In Harry Potter and the Philosopher’s Stone, when Neville gets a Remembrall, he says he doesn’t know what he has forgotten. If you look closely, he is the only student on the table who is in a sweater and has most likely forgotten his robes.
19. Snape’s first question to Harry in the Philosopher’s Stone in his potions class is: “What would I get if I added powdered root of asphodel to an infusion of wormwood?” Asphodel is a type of lily and in the language of flowers, it means ‘remembered beyond the tomb’ or ‘my regrets follow you to the grave’ while wormwood is often associated with regret or bitterness. This suggests that Snape deeply regrets Lily Potter’s death.
20. In Harry Potter and the Chamber of Secrets, when Hagrid charges into Dumbledore's office to defend Harry, he is seen carrying a dead rooster in his hands. In the extended cut of the movie, Hagrid mentions that someone is killing the school roosters. In the harry potter lore, the sound of a rooster is fatal to a basilisk. That’s why Ginny killed them under Tom Riddle’s control. It was also the rooster’s blood that was used to write on the walls.
In Harry Potter and the Chamber of Secrets, Lucius Malfoy attempts to cast Avada Kedavra on Harry after being tricked into freeing Dobby. It wasn’t explained in the movies at that point that this spell instantly kills a person. So Lucius was literally about to murder Harry in the middle of Hogwarts.
22. In Harry Potter and the Prisoner of Azkaban, a wizard played by Ian Brown from the band “The Stone Roses” is seen reading the book ‘A brief history of time’ by Stephen Hawking in the Leaky Cauldron, foreshadowing time travel that takes place later in the movie.
23. In the credits of Harry Potter and the Prisoner of Azkaban, the producers placed a naughty Easter egg in the Marauder’s Map. In one scene, two sets of footprints can be seen in the bottom left corner representing two people either having sex or making out. Also, when the Map is opened for the first time, the name 'Newt Scamander' can be seen. Scamander wrote the book “Fantastic Beasts and Where To Find Them” that is mandatory reading at Hogwarts, but he isn't a teacher there. Scamander eventually got his own movie with “Fantastic Beasts and Where to Find Them”, where it is revealed that he is friends with Albus Dumbledore, so in 'Harry Potter and the Prisoner of Azkaban', he may be simply visiting.
24. In Harry Potter and the Goblet of Fire, scenes featuring elves Dobby and Winky were cut due to time constraints. However, if you watch carefully in the first campsite scene, right after Ginny points to something and says "Look!" you can see two House Elves riding on llamas.
25. In Harry Potter and the Half-Blood Prince, while Horace Slughorn is disguised as an armchair, you can visibly see his feet.