The Alleged Expert: Gilderoy Lockhart was hired as the new Defense Against the Dark Arts instructor due to his track record of defeating magical monsters. His only real skill turns out to be stealing the credit from the wizards who really defeated those monsters.
Backwards Firing Wand: Ron's wand is damaged and becomes prone to firing spells through the wrong end, the one pointing towards the spellcaster... which is good news when Professor Lockhart steals it and tries to use it to erase Ron's and Harry's memories...
Bat Scare: Non-flying example: Huge crawling masses of spiders flee from Hogwarts. They do no harm, but badly frighten Ron, an arachnophobe.
Becoming the Boast: Subverted. Lockhart seems like a Know-Nothing Know-It-All with a grossly inflated ego. In reality...he's exactly that, plus amoral enough to steal other people's accomplishments and leave a child to die. When the staff suggest he go after the Basilisk, it's clear they're doing it to watch him squirm. When Harry and Ron *force* him to accompany them, since he's the closest thing they've got, he betrays them at the first opportunity.
Big Bad: Lucius Malfoy or the memory of Tom Riddle.
Bigger Bad: Voldemort does not personally appear in this book, but the conflict at Hogwarts this year is caused by his diary containing a fragment of his soul.
Break the Haughty: Lucius avoids prison time but suffers several blows to his ego. A twelve-year-old both stops his plan and tricks him into freeing Dobby, who takes the first opportunity to pay Lucius back for years of abuse by effortlessly blasting him down a flight of stairs. And Lucius also lost his job as one of the governors of Hogwarts, something he could have avoided if he hadn't threatened the other governors' families.
Cool and Unusual Punishment: Harry having to answer Lockhart's fan mail as the punitive detention he incurred for flying the car to school. (He doesn't see it as such, though.)
Contrived Coincidence: All the victims of the basilisk happen to see it indirectly, even taking into account those who are looking out for it.
Cover Identity Anomaly: Harry and Ron use Polyjuice Potion to pretend to be Crabbe and Goyle, but their infiltration of Slytherin House is stymied by the fact that they don't know how to get in to Slytherin's chambers. They ask a passing student, but she's from Ravenclaw. Fortunately, Crabbe and Goyle are so dim that they're not really acting out of character.
Critical Research Failure: In-universe — Tom Riddle seems to think that werewolves are much more animalistic than they actually are, considering that his false accusations towards Hagrid involved raising "werewolf cubs."
Dangerous Device Disposal Debacle: Ginny attempts to stop the attacks on the school by flushing Tom Riddle's diary down the toilet. Unfortunately, the toilet backs up and Harry and Ron find it, bringing it back to the Gryffindor common room and right back into Ginny's hands.
Darker and Edgier: While not as tense and brooding as the final books, Chamber of Secrets presents a huge leap in violence from the first book. More importantly, though, it introduces elements that are flat-out horror and can be argued not to have been surpassed even in the final books. Harry hearing the Basilisk's macabre ramblings while the monster stalks the piping system and the sinister, hostile message in blood aren't even the only examples.
Dead Guy Junior: Tom Marvolo Riddle, named for his father and his maternal grandfather. Or for both grandfathers, considering that "Tom" is usually short for "Thomas."
Didn't Think This Through: Harry and Ron when they steal Mr. Weasley's Flying Car. Granted, they're twelve years old at the time. Lampshaded by McGonagall when she points out other, more reasonable, things they could have done to get to Hogwarts, and Harry reluctantly admits to himself that stealing a car was pretty stupid.
Also, over the month they spent brewing the Polyjuice Potion to sneak into the Slytherin dorms, not one of the trio realized they have no idea where those dorms are.
Dinner with the Boss: Mr. Dursley's potential client and his wife come over to discuss a promotion over dinner. When Harry retires to his bedroom, Dobby the house elf appears and begs Harry not to return to Hogwarts. When he refuses, the house elf levitates a cake over Harry's head and drops it.
Disney Death: Ginny Weasley gets one in the Chamber of Secrets.
Does This Remind You of Anything?: Harry's (and Ginny's) interaction with Tom Riddle's Diary is extremely similar to that of an online chat room, as well as the part about the person being conversed with being revealed to not be trustworthy at all to begin with, a similarity made even more apparent in the film. It looks like the fear of every early internet user's parents: an unsuspecting kid is taken advantage of by someone they met online. Naïve Ginny pours out her soul “to an invisible stranger” she knows only through their text conversations and thinks she is making friends with this person. In reality he’s manipulating her, getting her to do things she normally wouldn’t, and when she goes to meet him in person she nearly ends up dead.
Harry has to save a girl by going into a chamber of secrets and fighting a giant snake that belongs to a very weird, much older guy that's been corrupting her for the whole book. With a legendary sword that holds strange powers. Uh-huh.
Do Wrong, Right: Arthur Weasley is far more pleased than his wife when his sons steal his flying car and use it to pick up Harry.
Eating Optional: When Harry visits Nearly Headless Nick's Deathday party, there's a banquet of rotten food laid out for the ghosts. Hermione figures they let the food rot so that the flavors are strong enough for the ghosts to actually taste.
Foreboding Fleeing Flock: Harry and Ron find a swarm of spiders leaving Hogwarts by an open window and heading for the forest. They later find out that the castle is home to a monster that spiders consider their mortal enemy: the basilisk.
"Maybe [Tom Riddle] murdered Myrtle; that would've done everyone a favor..." He did.
The first time Harry sees Tom Riddle's name, he has the strange feeling he knows Tom. He should: he's had a chunk of the guy's soul stuck to him since he was a year old.
A more subtle one, but Lockhart suggests that, on Valentine's Day, they ask Snape to teach them how to make a love potion, whereupon the narration remarks that Snape had an expression that clearly said that anyone who turned up asking for a love potion would be force-fed poison. In Book 6, Ron inadvertently consumes both a potent love potion and a lethal poison in rapid succession, though admittedly not from Snape.
Getting Crap Past the Radar: The reference to the Japanese golfer joke. The only Japanese golfer joke has the punchline, “What do you mean, wrong hole?
Hammy Herald: Fred and George show their support for Harry not by squashing rumors about him, but rather by following him around crying "Make way for the heir of Slytherin! Seriously evil wizard coming through!"
Heartwarming Moments: Subverted in-universe. What McGonagall thinks is is a CMOH is just the first lie Harry could come up with so he and Ron wouldn't get in trouble. Ends up being relevant to the plot because actually following through on the lie leads them to figuring out the mystery.
Hijacked By Voldemort: The book plays out like this. So a monster controlled by the Heir of Slytherin has been attacking students. At the end, we discover that -surprise!- Voldemort was behind the entire thing.
Hoist by His Own Petard: Ron, when trying to curse Malfoy ("Eat slugs!"), inadvertently curses himself, thanks to his broken wand. This is important later with Lockhart, hit by his own rebounding Memory Charm while using Ron's busted wand.
Holding It for a Friend: Filch claims that the Kwikspell letter on his desk is for a friend so that Harry doesn't guess that he has no magic.
Hot for Teacher: Hermione seems to one of many girls who have a crush on Lockhart.
I Never Got Any Letters: In the first chapter, Harry is upset that his friends haven't written to him all summer. Then it's revealed that an elf has been intercepting their letters.
I Never Said It Was Poison: Dobby inadvertantly reveals that he has been stopping Harry's letters when he mentions that Harry's friends haven't written to him — which is something he shouldn't be aware of.
Informed Ability: In-universe: Lockhart completely fails to live up to any of his hype, as Harry and Ron are quick to point out.
When wondering what Tom Riddle got a Special Award for Service to the School for, Ron sarcastically suggests that maybe he killed Myrtle as "that would've done everyone a favor." Riddle did kill her and he got the award for successfully framing Hagrid as her murderer.
Another potential example: Hermione suggests that whoever flushed Riddle's diary may have been the culprit, trying to stop anyone from finding out details about the last time the Chamber was opened. It was the culprit (sort of), but they did it to stop the attacks.
Harry wonders if, due to his speaking Parseltongue, he's a distant relative of Salazar Slytherins. It turns out that, through the Peverells, he is related to Voldemort, so he actually is, in some fashion, related to Slytherin.
Nice Job Fixing It, Villain: Lucius Malfoy planting the diary on Ginny Weasley. As we learn in the sixth book, this set off a chain of events that destroys one of Voldemort's Horcruxes and provides Dumbledore with the first real proof that Voldemort split his soul. Good going, Lucius.
Non-Malicious Monster: The basilisk might have been able to kill you only by looking at you. But it was controlled by "the heir of Slytherin" and didn't do anything unless he asked it to kill someone.
Noodle Incident: During the final confrontation in the Chamber, Diary!Riddle says that Dumbledore never seemed to like him as much as the other professors. Exactly what young Riddle did to trigger Dumbledore's suspicions won't be revealed until the sixth book.
Not Me This Time: Harry and Ron use Polyjuice Potion to imitate Crabbe and Goyle, Draco Malfoy's two Mooks, in the hopes of getting Draco to admit that he's the heir of Slytherin, and thus the cause of all the shenanigans happening at Hogwarts that year. Instead, they hear Draco ranting about how thrilled he is that it's happening and how he'd love to congratulate whoever is actually behind it.
Portal Slam: Harry and Ron rebound painfully from the portal to Platform 9¾.
Post Mortem Comeback: The entire basis of the plot; Voldemort hid pieces of his memories in a book, who took the form of Tom Riddle, but it's inverted — Voldemort himself was already alive to begin with... somewhat.
Predecessor Villain: Salazar Slytherin was the guy who built the Chamber of Secrets and put the Basilisk there in the first place, but he's been dead for nearly a thousand years when the story begins.
Punny Name: Knockturn Alley, like its good counterpart Diagon Alley, is this ("Nocturnally" or "Nocturne" Alley).
Ransacked Room: Ginny ransacks Harry's dormitory in order to get Tom Riddle's diary back.
"The Reason You Suck" Speech: Harry gives one to the memory of Riddle, gloating over how his Muggle-born mother reduced Voldemort to almost nothing.
Red Herring: Percy, Hagrid and Malfoy — and Harry — are all set up as possible Heirs of Slytherin, with Malfoy being the choice that's so obvious it's stupid, and Percy and Hagrid both having Really Big Secrets that make them act suspicious. Naturally, Malfoy is the one our heroes suspect and they spend half the book finding out that it isn't him.
Reptiles Are Abhorrent: Basilisk. Also the decoration of the Chamber of Secrets. Salazar must have been swapping design tips with Jafar.
Right Behind Me: Harry and Ron excitedly speculate on reasons why Snape is absent at the beginning of the year, not realizing that Snape is actually just behind them.
Ron also suggests that Dobby might belong to the Malfoys, and that his whole spiel about "protecting" Harry is just Draco's trick to get him sent home. Dobby does belong to the Malfoys, but his motive is legitimately good.
Spider Swarm: The giant spiders (also known as Acromantulas) appear to be social, although instead of a queen, they are led by an elderly male spider named Aragog. It's explicitly stated that Aragog had a bride, so apparently, Acromantulas females don't feed on their male.
Stealing The Credit: Lockhart, on a fairly horrifying scale. Be careful if you're a witch or wizard who has done something either brave or impressive. He doesn't just steal the credit, he wipes your memories.
Villain of the Week: Subverted. Up until the last couple of chapters it looks like the series will be heading this way, with the Big Bad Voldemort introduced in the first book and the mysterious Heir of Slytherin being the main villain in the second book. Near the end however it's revealed that not only is Voldemort the Heir of Slytherin, but a memory of his younger self has been driving the plot the entire time. Starting with the third book, all of the main antagonists (except Umbridge) are explicitly linked to Voldemort.
Weaksauce Weakness: The Basilisk would die at the crowing of a rooster (presumably because it is hatched from a chicken's egg incubated by a toad). One wonders just how effective a weapon a Basilisk would be to a dark wizard in any other situation, given everyone knows this and roosters aren't exactly hard to come by.
"Your Bludger?" said Harry, anger rising once more. "What d'you mean, your Bludger? You made that Bludger try and kill me?" "Not kill you, sir, never kill you!" said Dobby, shocked. "Dobby wants to save Harry Potter's life! Better sent home, grievously injured, than remain here, sir! Dobby only wanted Harry Potter hurt enough to be sent home!"
What Measure Is a Non-Human?: Those poor mandrakes. Even though everything suggests that they are sapient, social lifeforms (they get "moody and secretive" in their adolescent phase, enjoy partying, and when they reach adulthood, start moving into each other's pots), nobody seems to think chopping a mandrake up for a potion is any different than chopping up a carrot.
Who Would Be Stupid Enough / Description Cut / Gilligan Cut: At the very end of Chapter 9, "The Writing on the Wall", Hermione comes up with the idea of using Polyjuice Potion to get information from Malfoy. However, the book with info on how to make it is in the Restricted Section of the library, and they need permission from a professor to take out the book, which would raise suspicion from any professor they ask.
"Oh, come on, no teacher's gonna fall for that!" said Ron, "they'd have to be really thick..." [end chapter] [start of next chapter] Since the disastrous episode with the pixies, Professor Lockhart had not brought live creatures to class...
Wrote the Book: Inverted; even though Lockhart literally wrote the book on dealing with magical pests, that doesn't mean he has a clue about it.
Xanatos Gambit: Lucius Malfoy's plan has two possible outcomes: either Ginny is caught, thus disgracing Arthur Weasley and his Muggle Protection Act, or the culprit is not apprehended, and either kills every Muggle-born in the school or drives them all away. The former seems to be his preferred option, interestingly enough, but either would presumably satisfy him. Of course, his plan backfired worse than he could have possibly imagined, which we learn in Deathly Hallows.
You Are Better Than You Think You Are: Harry feels unworthy of his fame as well as his house, Gryffindor. Dumbledore manages to assure him that he does belong in Gryffindor, showing Harry the sword as proof.