1.First, on Ravenclaw's table in The Great Hall, you can spot a box of "Cheeri Owls" cereal — a nod to Cheerios.
2.In The Sorcerer's Stone, school children wearing green school uniforms walk by the reptile room. This is a nod to Slytherin's house color being green and their symbol being a snake.
3.When Dumbledore visits young Tom Riddle in The Half-Blood Prince, Tom has seven rocks on his windowsill. This foreshadows Voldemort's seven Horcruxes.
4.Also, Harry's Quidditch number in The Half-Blood Prince is seven.
5.In addition, seven locks appear on the chest Barty Crouch Jr. keeps the real Mad-Eye Moody in during The Goblet of Fire.
6.In young Tom Riddle's room in The Half-Blood Prince, you can spot a picture of the cave where he eventually hides one of his Horcruxes.
7.In The Deathly Hallows Part 2, you can spy Lucius Malfoy's Azkaban prisoner number tattooed on his neck.
8.During The Prisoner of Azkaban credits, you can see two pairs of feet on The Marauder’s Map that appear to be having sex.
9.Speaking of The Marauder’s Map, during The Prisoner of Azkaban, Newt Scamander from Fantastic Beasts and Where to Find Them can be spotted.
10.In The Chamber of Secrets, Sherbet Lemon is the password into Dumbledore's office. Then, in The Half-Blood Prince, the candy can be seen on Dumbledore's desk.
11.In The Sorcerer's Stone the Gryffindor Quidditch team plaque shows that Harry’s dad — and possibly a relative of Professor McGonagall’s named M.G. McGonagall — also played.
12.In The Chamber of Secrets, you can spot Gilderoy Lockheart's second wig on his desk.
13.In The Prisoner of Azkaban, a wizard sitting in The Leaky Cauldron is reading A Brief History of Time by Stephen Hawking — the wizard is played by Ian Brown, the lead singer of the band The Stone Roses.
14.In The Sorcerer's Stone, when Hagrid takes Harry to The Leaky Cauldron for the first time, you can see the sign become visible as they approach the door — the sign is hiding from Muggles on the street.
15.In The Deathly Hallows Part 2, you can spot one of the chess pieces from The Sorcerer's Stone in the Room of Requirement.
16.In The Chamber of Secrets, Hermione has a chameleon on her desk. This is foreshadowing when Polyjuice potion is used later in the film so that Harry, Ron, and Hermione can blend in with Slytherin students.
17.In The Sorcerer's Stone, you can see that everyone in the Dursleys' neighborhood has the same car. This makes everyone, including the Dursleys, appear as normal as possible.
18.In Order of The Phoenix, one of the Daily Prophet newspapers shown actually has a weather forecast.
19.Finally, as the movies progressed and the stories got darker and darker, so did the Warner Bros. logos that began each film.