CAUTION:Read this article carefully before using it — if you don't, you'll probably mistake it for Not a Subversion.
Basically, this is playing bait and switch with a trope. A work makes you think a trope is going to happen, but it doesn't.
But how could people know a trope is going to happen? Well tropes live in the minds of the audience. As such, sufficiently Genre Savvy (or Trope Savvy) audience members can predict a familiar trope coming based on the hints dropped by the writer. So when the writer decides to build on this expectation, only to reveal that the expected "trope" was a Red Herring while an entirely different situation results, you have a Subverted Trope.
Phrased another way, the work is ultimately revealednot to be using the trope at all, but in the meantime was played up to look like it was.
This is one method of leveraging a trope to give a story texture. It certainly isn't the only way.
A subversion has two mandatory segments. First, the expectation is set up that something we have seen plenty of times before is coming, then that set-up is paid off with something else entirely. The set-up is a trope; the "something else" is the subversion.
To put this another way, a trope of the form "X are often Y" is not subverted by every X you can think of that isn't Y. If someone is murdered and there's a butler around, but he didn't do it, that's not automatically a subversion of The Butler Did It. But if the writer makes it look like a typical example of The Butler Did It, then reveals he didn't, that's a subversion.
A full comparison could go something like this: A car chase is in progress at reckless speeds. The camera cuts to some workers carrying a Sheet of Glass, then cuts back to the panicked driver headed towards the workers. It seems pretty obvious that the driver is going to smash the glass sheet into a million fragments... or is it?
If the car drives through the pane of glass, it's played straight.
If the car drives through the pane of glass, and the workers are heard complaining about why cars that are being chased can avoid nearly everything but a pane of glass, it's lampshaded.
If the car misses the pane of glass, it's subverted.
If something else causes the glass to be broken before the car can even make it to where the glass pane broke, it's also subverted.
Another subversion is if the car hits, but somehow the car phases through and neither the glass nor the car is broken.
If the car misses the pane of glass but something else causes the glass to be broken, it's a double subversion.
If the car hits the pane of glass, and the result is that the glass merely has a car-shaped hole in it, that's downplayed (and also Played for Laughs, but that's another matter. It's also Impact Silhouette played straight.)
However, if the car hits the pane of glass, and the result is that the glass merely has a car-shaped hole in it, but the pane of glass collapsed on itself, it's either played straight or a double subversion (And also breaking a downplay).
If the car hits the glass, but it's the car that shatters (instead of the glass), it's inverted (and a very shoddily-built car at that).
If there is no pane of glass at all, it's averted.
Conclusion: when posting examples, remember that just not doing a trope isn't the same thing as subverting it. Unfortunately, most tropers tend to forget that.
In addition, kindly avoid using "subverted brutally" or "subverted hard" when describing the manner in which a trope is subverted. There is no guarantee that your example was subverted any harder than the previous one. The same goes for saying that a trope is "kinda subverted", as treating tropes to play and subversion is by definition inexact.
Bear in mind that, just as Tropes Are Not Bad, subversions are not automatically good, witty, clever, or original; conversely, don't hesitate to add a subversion (that's actually there) just because you think the work is inane and stupid.
Meta Trope Intro compares this with many other ways that a trope can be used.
See also Discredited Trope, Dead Horse Trope, Double Subversion, Downplayed Trope, Playing with a Trope.
Every trope page has 'subverted in...' somewhere on it. Please, apply the Wiki Magic!
Cross Game: A beautiful girl is attacked by three delinquents. But lo, a hero arrives! With but three mighty punches he decks the villains, saving the girl. Noticing her injured hand he gentlemanly offers to provide medical treatment at his conveniently nearby home...
Wait a minute... we've seen this hero before. Gasp! He was part of the gang of delinquents — it's an Invoked Trope! Oh dear, what will happen to the beautiful innocent girl now? Will our onlooking heroine warn her in time?!
Wait... what is the beautiful innocent victim doing now? She's got her cell phone out! She's calling the police! "That was extortion they tried to commit — the police need to know about it." The "delinquents" flee, revealing the truth.
Rurouni Kenshin has a degree of subversion of the Determinator trope, in how he reacts when he's truly put into a dangerous fight. The layers of his friendly, pacifistic personality start to fall away, until beyond a certain point he becomes the cold-blooded killer he once was.
In Sangatsu No Lion, Rei's name uses the kanji for zero and nothingness. However, My Hero Zero is subverted when it just gives his adopted sister Kyoko fuel to taunt him with, as it is not played for Rule of Cool.
Kyoko: You're name is Rei? What a weird name! But it suits you ... "No home. No relatives. No school. No friends."
A series of Discover credit card ads established a pattern of people calling their customer service line, and having their queries answered by an employee who symbolically looks just like them ("We treat you like you'd treat you"). Then they subverted viewers' expectation that the employee's resemblance was purely symbolic, having the employee in a new ad be the caller's actual identical twin.
Watchmen: Adrian Veidt is set up to be the ultimate in Ambiguously Gay, with all of the preening attention to his own physical appearance and lack of fighting ability that is associated with that trope. Then somebody tries to shoot him, and he responds by picking up an eight-foot-tall floor lamp and using it to bash the gunman into a fountain, then climbing in after him and demanding to know who sent him while shaking and choking him. Subverted even further when we find out that Veidt hired the assassin himself as a red herring, and when Veidt proceeds to beat the everloving crap out of Nite Owl and Rorschach simultaneously, and then proves his skill further by catching a bullet.
The ending can be seen as a major subversion Rorschach and Nite Owl confront the now realized-to-be-the-evil-villain Adrian, he explains his master plan and when told that he will be stopped, he informs them that he carried out his plan before they even got there.
In the season 2 finale of Carnivàle, Jonesy strikes Varlyn Stroud unconscious with a log of wood. He then runs into the house that Varlyn was about to enter and rescues Sophie, leaving Varlyn and Varlyns handgun unattended right outside the door. Seconds later, Josey gets shot... By Sophie.
This is a staple of much of the comedy on Mongrels; starting out with what seems to be a buildup to an obvious joke only to quickly turn it around (often lampshading it in the process), like so:
Nelson: How did you get these documents?
Badger: Let's just say I have a... "mole" on the inside.
(cut to an ordinary-looking person in an office, grabbing documents off a table and sticking them into an envelope)
Nelson: Huh. Y'know the way you said that I was expecting an actual mole.
Badger: Nope, he's a person.
Zyuden Sentai Kyoryuger had a Monster of the Week named Debo Hyogakki. He was credited with having wiped out the dinosaurs, but being the first opponent that the team faced, it seemed to be an Informed Ability. Then we got episode 17, where it was revealed that the monsters were getting stronger as Deboss was reviving, to the point where the Kyoryugers started having trouble with the Mooks until they got their Full Potential Upgrade. Then episode 21 subverted it even further by revealing when he was revived that Hyogakki wasn't the only one who wiped out the dinosaurs. He, along with Debo Viruson and Debo Nagareboshi, were a Power Trio known as the Zetsumates, a pun on the Japanese word zetsumetsu, or extinction. Then in episode 25, Hyogakki, as the last of the Zetsumates, showed his true power by making it so that if the people in the city shed a single tear, they'd turn into a Human Popsicle. So while you might have chuckled at his reputation when you first met this guy, it ultimately is revealed that he wasn't nearly as weak as he initially seemed.
Lemon Demon's Being a Rock Star handily subverts the Rockstar Song trope. It starts with some generic lyrics about rockstardom, but rapidly switches to insulting the concept in the space of a verse or two.
P.D.Q Bach's "Concerto for Horn and Hardart" contains a subversion of the Theme with Variations.
Peter Schickele: The striking thing about the middle movement, the Theme with Variations, is that the variations have nothing whatsoever to do with the theme. Now, that's one of those things that everybody takes for granted, but why not? I mean... This is apparently variations on some other theme. Perhaps we'll turn that other work up someday.
Blur the Lines subverts Stuffed into the Fridge when Drew receives a present that the giver implies is the head of his lover, Rick. After crying out because he thinks his lover is dead, Rick walks up behind him and says, "What? Why are you screaming?" 
Superman: The Animated Series plays straight and subverts many tropes, but it is noteworthy the way it handled Bury Your Gays. Maggie Sawyer was primarily a victim of Hide Your Lesbians, up until the second season episode Apokolips...Now! there had not been any hints to her orientation in the comics, but she seemed to be legitimately set up for death. When she is attacked by Intergang she is thrown from her car in a fiery explosion and she is shown horribly burned beneath a crushing pile of rubble, noticeably withouta blinking eye or moving fingers. It looks like she is Killed Off for Real, especially when Dan Turpin starts calling the attackers "murderers" while screaming at them, except a later scene reveals her to be Not Quite Dead. This later scene, which revealed that she had survived and thus subverted the Bury Your Gays trope, also provided the first ever hint at her sexuality when she is visited in the hospital by a woman that the DVD commentary reveals is Toby Raines, her partner in the comics. So, Maggie not only survives the attack, which is a straight subversion of the Bury Your Gays premise, but the setup actually lead to a (partial) revelation of her sexuality, something this trope is usually invoked specifically to avoid, making it also an inversion.
Burns: Of course not. The very notion of a talking banana is absurd. But still....
In Vino Veritas is subverted in the episode "Mountain of Madness," where a park ranger enters a cabin and finds it full of partying employees from the nuclear plant.
Ranger: Hey, what is going on here? Who are you people? This is a lookout post. Where is Ranger Mc Fadden?
Drunk: I was just happy to see so many nice people!
Ranger: Quiet, you drunk. Where is Ranger McFadden?
(The camera then moves a step to the side, revealing a straight-laced ranger with glasses)
Ranger McFadden: Right here, sir, behind the drunk.
A third example, also from "Monty Can't Buy Me Love," is when Mr. Burns, Homer et al have finally found the Loch Ness monster, who proves impossible to subdue. Finally Mr. Burns walks toward the monster with a stern look in his face. We expect an epic fight where Mr. Burns is revealed as a Badass Grandpa handing out an unexpected ass-kicking — but instead the scene cuts to the team's helicopter in the air, with Nessie tied up and swinging below. Mr. Burns explains to the admiring team:
Burns: I was a little worried when he swallowed me, but ... well, you saw the rest.
Another episode subverts the Sheet of Glass example mentioned above. In this case, the car hits the glass, but simply knocks it down flat on the ground and drives over it. The workers then pick the glass back up noting "wow, tough glass."
The glass example is subverted again in the episode where Bart gets an elephant. The elephant runs off, stampeding down a street towards two workers carrying a glass pane. They jump out of the way of the elephant, with the glass surviving, only to jump right into the path of a skateboarding Bart... Who they also successfully avoid, eventually making it all the way across the steet, glass in tact, to complete there goal of throwing it into a dumpster, shattering it to pieces.
The Simpsons really loves playing subversions for laughs. Another example: When the Simpsons are kicked out of their house, Homer remarks: "Well, at least it'snot raining!" Beat. "See, it's not raining."
In Family Guy, Peter has just launched himself from a cannon. Cut to a living room:
Guy: Great, I've got all my dominoes set up exactly how I want them, next to the good china. Now I'll just place this priceless faberge egg next to my newborn hemophiliac baby....
(Peter lands with thud outside the window, and looks in.)