31Tom Riddle's Diary
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.
32Fred and George
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.