26Dobby and Winky
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.
27. In Harry Potter and the Half-Blood Prince, while Horace Slughorn is disguised as an armchair, you can visibly see his feet.
28. When Dumbledore visits a young Tom Riddle in Harry Potter and the Half-Blood Prince, there is a picture of the cave where Tom hid one of his Horcrux. There is also a brief close up that shows 7 rocks meticulously organized on Tom’s windowsill, foreshadowing Voldemort’s plan to split his soul into seven Horcruxes.
29. At the beginning of Harry Potter and the Chamber of Secrets, Lucius Malfoy takes a book out of Ginny’s cauldron but when he puts it back you can see that he puts Tom Riddle's diary in the cauldron as well. There is even an inkling of confusion in Harry's face but he doesn't quite understand why. This detail is explained at the end of the movie, but people usually miss it at the beginning.
30. Throughout the series, Weasley twins Fred and George are always shown in that order on the screen, left to right, so that audience can easily tell them apart.
