Accidental Innuendo: "Too true," muttered George Weasley. "I haven't been properly dry since August."
Adaptation Displacement: Lucius Malfoy's appearance is barely described in the book, beyond looking like an older version of Draco. The long hair, velvet cloak and walking stick to conceal the wand were all thought up by Jason Isaacs, and are generally accepted as canon.
Alternative Character Interpretation: Arthur Weasley, patriarch of the Weasley clan. Is he a loving husband and father whose Muggle-studying hobby is just an out-of-control quirk? Or is he a selfish man, clinging to a job that doesn't pay enough for him to provide adequately for his large family, just for the sake of greater license to indulge his fascination with Muggle culture? Goblet of Fire adds to the confusion when it's revealed that the hobby is specifically why his bosses refuse to promote him. it's also why Percy Weasley becomes the Token Evil Team Mate and abandons his family between Order of the Phoenix and Deathly Hallows.
According to Wikipedia, several Christians — those who don't think that these books teach Satanism — compare the climax to John Bunyan's The Pilgrim's Progress, a pinnacle of Christian literature.
In his relationship with Ginny, it's easy to see Tom Riddle as a kind of metaphorical Internet predator. J. K. Rowling herself acknowledged in an interview on the DVD of the movie that the diary is really a lot like an Internet chat room, but said she hadn't been in one at the time she wrote it so it's just a coincidence. Rowling has also said her inspiration was the fact that she found diaries to be really scary, as a person's deepest darkest secrets are hidden in them. So rather than the focus of the danger being on talking to strangers, it's more on playing with something you don't understand.
Although Gilderoy Lockhart's memory being wiped is sort of Played for Laughs, it becomes horrifying when we see him again in book 5 in the Spell Damage Ward at St. Mungo's. Also interesting, considering that the memory charms that Gilderoy Lockhart put on the people who actually did the deeds he claims he did are decidedly not played for laughs.
There's a comedy sequence of the Weasleys constantly being held up on the way to King's Cross. One of the delays is Ginny stopping the car to get her diary. Had they not gone back for it, the whole thing wouldn't have happened.
Genius Bonus: The bronze statue of a boar that has a prominent place in the Hogwarts entrance hall is a copy of Il Porcellino, a famous statue in Florence, said to have magical powers. The original Il Porcellino figures in one of Hans Christian Andersen's lesser known fairy tales, The Metal Hog, in which it comes to life and tells a poor street urchin that he is destined to become an artist — not unlike what happens to Harry Potter.
Harsher in Hindsight: The story behind Hagrid's expulsion from Hogwarts has become even more disturbing now that it's been revealed that something very similar happened to Newt Scamander. Even if one were to accept that no one believed in the legend of the Chamber of Secrets, how could you expect anyone who didn't know the real story to feel sympathy for Hagrid after the second monster attack in living memory — and this time it was actually deadly?
Heartwarming in Hindsight: All of Harry and Ginny's interactions, when you know they end up together in the end. It's hard not to smile when rereading the scenes where Ginny blushes over him, Harry saves her, or they stick up for each other. These two have no idea what's in store.
Remember when Harry was less than fond of the Skele-Gro? Turns out his ancestor, Linfred of Stinchcombe, invented the stuff ages ago.
Snape looks "as if Christmas had been cancelled" when he finds out that Harry and Ron won't be expelled for flying the Ford Anglia to Hogwarts. He would later be portrayed by Alan Rickman in the films, who himself called off Christmas as the Sheriff of Nottingham in Robin Hood: Prince of Thieves.
The Reveal that Lockhart's "memoirs" are full of bullshit. Within ten years of the release of the book, similar reveals occurred regarding two memoirs (A Million Little Pieces by James Frey and Love and Consequences by Margaret Seltzer) that Crackedhas summarized as "lame-ass white kid wants to be a stone-cold criminal." In fact, Margaret Seltzer claimed to have used Lockhart's method of taking other people's life stories and passing them off as her own. A little after that, a Holocaust memoir was revealed to have been forged, but that arguably falls under Harsher in Hindsight or "Funny Aneurysm" Moment.
Among the people who auditioned for the role of teenage Tom Riddle was one Eddie Redmayne, who apparently bombed hard. Fifteen years later, he's back in the Potter franchise in a big way as Newt Scamander.
Shirley Henderson gets cast as Moaning Myrtle. She had just starred in the film version of Bridget Jones, where her first scene featured her crying in a bathroom.
The Skele-Gro scene became a lot funnier than it was meant to be after the "bone hurting juice" meme became popular.
When Ginny stands up for Harry after Malfoy taunted the latter, Malfoy jokingly remarks that "[Harry's] got [himself] a girlfriend". Four films later, Harry and Ginny have a Relationship Upgrade.
At one point in the film, Ron attempts to turn Scabbers into a goblet only for him to turn him into a rat/goblet hybrid. With the reveal at the end of the next installment, one can only wonder how Peter Pettigrew was reacting to the failed spell.
It's the Same, Now It Sucks!: Many film critics talk down to this film for taking a slow-paced, Adaptation Distillation approach like the first film. After the third film came out and the tone of the film series changed profoundly, it's common for only purists to mention this one. The book is also given similar treatment.
Lockhart is a fraud. This was fairly obvious at the start, but the reveal that he memory-wiped the real heroes was a surprise on the first reading (if only because before then, he came off as laughably incompetent and a huge Jerkass, but otherwise harmless).
Jerkass Woobie: Filch qualifies when his cat, Mrs. Norris, gets petrified. Also when it turns out he's unable to perform magic despite being wizard-born. No wonder the guy hates Hogwarts students so much.
In the film, Hermione saying "fear of a name increases fear of a thing itself" and "even in the wizarding world, hearing voices isn't a good sign" prompted fans to erupt with fury that she was being given other characters' lines. In the books those are said by Dumbledore and Ron respectively.
This film is usually the one attacked for the Wimpification that happens to Ron, notably playing his fear of spiders more for laughs than drama.
J. K. Rowling received several angry letters from readers who weren't able to finish the novel because they were too scared to go on reading.
The serpent's voice is quite disturbing in the audiobooks.
The more you think about it, the more disturbing the things Riddle does to Ginny become. Especially when he mocks her genuine and understandable fears to Harry.
The chapter art for the polyjuice potion sequence, which resembles one of the middling stages of an Animorphs cover as Harry transforms.
"Seinfeld" Is Unfunny: Upon its release, the film adaptation was almost universally praised as an Even Better Sequel for its darker tone, improved effects, and better performances by the child actors compared to the first film. As the later films got even darker and improved on the other aspects, people began to see this film as more tame and repetitive of the first.
Signature Scene: Literary example, although the film version remains faithful to the scene in question: to this day, Dumbledore's exposition to Harry near the end of the book is considered one of the most iconic of Dumbledore's speeches in the series.
In the film, when Harry wields the Sword of Gryffindor against the basilisk, there are several points where one can tell that Radcliffe was not wielding a sword so much as a ball-pointed prop.
In the shot where Harry crashes his cart after unsuccessfully trying to enter Platform 9 3/4, Hedwig is briefly replaced by a very obvious owl dummy.
The Quidditch matches in general tend to be full of conspicuous CGI, but Chamber of Secrets has a particularly cringeworthy shot where Harry flies through the Slytherin stands, causing the students there to have to jump out of his way. The actors are obviously digitally inserted into the scene and move in an incredibly unnatural way.
Squick: Harry getting his arm broken. Lockhart tries to fix him up, and the bones disappear. In the film version all that's left is a misshapen mess that makes an unnerving sound.
They Wasted a Perfectly Good Character: One Mugglenet book suggested the basilisk to be this. It's apparently intelligent, centuries old, and Harry can understand its speech, but it only shows up at the end and mostly acts like a generic monster, rather than giving the audience any signs of even being a character.
Uncanny Valley: Hermione when she's briefly turned into a cat. The fact that the cat head is human-sized does not help the creepiness.
Visual Effects of Awesome: The Fawkes animatronic. To put it into perspective, when Richard Harris first saw it, he initially thought it was a real bird and was impressed with how "well-trained" it was.
In the film, Lucius Malfoy attempts to cast the killing curse on Harry right outside of Dumbledore's office. Blame Jason Isaacs for that one: when they filmed the scene, he ad-libbed the first spell that came to his mind, and the books were only up to Goblet of Fire at the time.
Hermione mistaking a cat hair for a human hair leading to her unfortunate Polyjuice Potion accident — how can the smartest witch of her generation be dumb enough to mistake a cat hair for a human hair when considerably less intelligent people can tell the difference with ease?
Vernon Dursley. Do you not remember what happened the last time you tried to keep Harry from going to Hogwarts? He's lucky he only got three Weasley boys. Imagine if Hagrid — upset when giant man-eating spiders are harmed, mourn the loss of his baby dragon fiercely, three-headed dog raising, refuses to leave his literal-giant brother behind because the other giants are mean to him, Hagrid had found out that the Dursleys were locking Harry in his room and starving him. Exactly how many pieces of Vernon do you think would have been left?
Ron and Harry. When they can't get to the platform, instead of waiting for Mr. and Mrs. Weasley to return (or any wizard, really) and help them get to Hogwarts, they steal the car and try to get there on their own. Justified in that they're 12 and slightly panicked. McGonagall actually points this out later, asking them why they didn't just write to her or Dumbledore and explain the problem. Harry privately realizes that that was an obvious solution that he didn't even think of.
Poor Ginny, once everything that's happened to her is revealed. Part of the reason Riddle was able to prey on her was because she was so lonely, and prone to being overlooked by her family. He became her only companion, so the fact that he turned on her is even more horrifying. And she's only eleven too.
Voldemort's Significant Anagram name, revealed in this book, in the original was Tom Marvolo Riddle, an anagram of "I am Lord Voldemort." Translations changed various parts of his name; for example, in the German version, his name was Tom Vorlost Riddle, which becomes "... ist Lord Voldemort" (is Lord Voldemort). Something is gained in the German version particular here, as his middle name sounds an awful lot like Verlust, meaning "loss," which applies to Voldemort in a variety of ways.
The Spanish translation called it Tom Sorvolo Ryddle, to spell Soy Lord Voldemort. Of course, "Ryddle" isn't an English word, but a Spanish speaker could pronounce it the same way as the English name without noticing anything wrong.
Other languages aren't quite so lucky. Just ask Tom Elvis Jedusor, of the French translation.
Although "Jedusor" (read "jeu du sort", i.e. "game of chance/fate/spell" carry a signification interesting enough.
Or Romeo G. Detlev Jr. in Denmark (The G. stands for Gåde = Riddle)
Russian translation got interesting with it: since the first two books were translated back-to-back, Tom's name had only two letters changed (and one removed since that's what Russian language does to silent e's) — becoming Tom Narvolo Reddl. It's Voldie's name that got changed — to Volan-de-Mort. And thus Tom Narvolo Reddl becomes Lord Volan-de-Mort. It creates a small continuity error in the sixth book, in which his and Marvolo Gaunt's name get their M back.
In Hungarian it became Tom Rowle Denem. "De nem" is negation in Hungarian, and the anagram became "Nevem Voldermort" meaning "(My) name is Voldemort", putting Voldemort disowning of his father's name into both said name and into the anagram.
Ironically, The Bulgarian translation was a bit hard to do because of a translation done ''too well'': Back in the first book, Voldemort's name was transliterated as Волдемор instead of Волдеморт* Modern Bulgarian rules for transliteration dictate that it follows a word or name's pronunciation in the original language; the translator correctly deduced that Voldemort's name comes from the French mort where the T is silent. The lost T meant that in the anagram, the T from Tom's name had nowhere to go if one were to think of some "I am Lord Voldemort" variant. Together with the I* "И" [i] means "and" in "Riddle", that resulted in the rather awkward "Тук съм и Лорд Волдемор", which could be taken to mean "Here [in the Chamber of Secrets] I am Lord Voldemort as well".