There's an interesting anecdote about John Dies at the End regarding Camel Holocaust, the "song" that John wrote for his band early in the book. In the original webnovel, the protagonists have to stall a group of monsters at a later date by playing Sweet Child O' Mine on a set of guitars they stole from Elton John. When the book was to be put into print, however, the issue of copyright came up. The author stared dumbly into space, scratched his butt, and realized that he had left Chekhov's Gun sitting in his back pocket. Thus, the day was saved by Fat Jackson's Flap WagonThree Arm Sally.
The Whateley Weapons Fair at Whateley Academy in the Whateley Universe. Phase is asked to try a forcefield disruptor by an inventor who has very little cred. It's just the thing Phase needs at the end of the Fair, when someone's weapon makes everything else go haywire. Then, much later, Phase uses another one of the forcefield disruptors in a fight, and it blows up on her.
The Weapons Fair is turning into a Chekhov's Armoury. The attack devise in Knick-Knack's 'lava lamps'? Used to attack Phase in a much later novel. Phase's run-in with Kew and the Spy Kidz? Important in "Ayla and the Networks". There seems to be a lot of these.
Face it, the Phase novels are nothing but Chekhov's Guns. Some of the background issues in Ayla 1 turned out to be key plot points in Ayla 6. Phase's concern with the New Olympians in Ayla 4 turns out to be critical in Ayla 7. Delta Spike's rambling in Ayla 7 about a course she took last year turns out to give Phase a crucial clue early in Ayla 8. And those are just some of the ones where the gun doesn't get fired in the novel where it's shown.
The web series commodoreHustle (by the guys at loadingreadyrun) introduced Mr. Ballsmatron in episode 7, and other than a few cameos, it never played a role until the season finale, with an ultimate ball kick and its destruction. Making it possibly the first appearance of a Chekhov's Ball-kicking robot.
Boatmurdered, a well-known succession game of Dwarf Fortress, has an example - an early ruler builds a catapult in the souther parts of the outdoor plains to take out problem elephants, get rid of surplus stone and train siege operators. Due to a lack of manpower and constant attacks, it never sees use and isn't even mentioned again. When a later ruler allows magma flow from Project Fuck The World to reach the southern parts of the map, it sets the catapult on fire. The smoke clouds and spreading blaze from that one structure ultimately lead to the fortress's downfall.
Then there's the later succession game Headshoots, and The Inexplicable Room and the path to it. Nobody remembered making either, no ruler could find how it connects to the rest of the fortress, and it could only be found via the 'find unit' function. Even a majority of the dwarves couldn't find their way in or out. The final turn, where the two unbelievably-strong soldiers Holistic Detective and Nemo2342 were skeletonized and sent against the entire fortress, the only surviving dwarf hid in the parth to The Inexplicable Room.
At the beginning of Just Another Fool, there's a random vignette about Logan's watch. Later, it becomes a major clue for a puzzle.
Mocked in The Nostalgia Chick's review of Showgirls. During Nella's song at the end, she mentions the Chekhov's Stairs (that the lead pushed someone down at the climax of the film) that have been there since Act One.
In one Global Guardians universe story, Knightblade's Cool Car makes an appearance at the beginning, but then is promptly abandoned as the manhunt moved into an abandoned tenement. At the end of the story, Knightblade was forced to use his Cool Car as a weapon against a supervillain when he finally chased him out of the tenement.
Linkara's Magic Gun in Atop the Fourth Wall, which he's been wielding since the beginning, turns out to be rather important in the Silent Hill: Dead/Alive review, going from prop to plot point and character.
In his review of a World Of Warcraft Comic, Linkara finds a working pokeball. He later uses it to capture a pyramid head
In this episode of College Humor's Hardly Working, David casually references a book that Sam borrowed, and never returned. This is largely ignored until the end of the episode, when David realizes that Sam was trying to kill him so he wouldn't have to return the book. He even goes so far as to Break The Fourth Wall by looking at the camera and saying "Remember? From the beginning?"
In I'm a Marvel... And I'm a DC, Spiderman telling Batman that he thinks he might have been married once would help Batman realize how Spiderman was immune to the Joker's brainwashing.
In an early Fine Structure story, Seph thinks to herself that Mitch, who can phase through things, could kill someone instantly by materializing his hand inside their head, but she doesn't think he'd do that and doesn't mention it. Near the end of the series, he does just that.
In Four Swords Misadventures, Green Link procures a broken magic mirror, thus getting them into a series of misadventures before finally warping back to the entrance of the first dungeon. Afrerwards, Red decides to dipose of it, thinking that it would do them more harm than good. Turns out, the mirror was the only way barring an extensive trading quest for the heroes to get to the Twilight Realm where Zelda is being held by Vaati. In other words, Red really should have let Green keep the mirror so it could be fixed.
Both subverted and parodied in one episode of Ashen's Tech Dump when, after showing off a highly toxic action figure sealed in a glass case with a biohazard sticker on it, he blatantly sticks it so that it's balancing precariously on the front of the desk and continues with the episode. When the case spends the whole episode without falling off, he finally just reaches out and pushes it off the desk himself.
At the beginning of Eliezer Yudkowsky's article "The Simple Truth" the author uses an analogy to advise people not to abandon the concept of truth, using the example that even when gravity wasn't understood, people had the common sense not to walk off cliffs. By the end of the story, Markos Sophisticus Maximus denies the concept of truth, and walks off a cliff.
At the start of Greek Ninja, Sasha mentions having odd dreams, which although it's implied there is some importance to them, they are otherwise dismissed, until it's revealed in the fourth chapter that they are actually memories from Sasha's previous life as Eli of Thrace.
TV Trash: The Mighty Ducks review begins with Hewy Toonmore (from Hewys Animated Movie Reviews) breaking into Chris's show by using a remote, mentioning that all online reviewers have this. This is used as a chekov's gun twice. Chris would later obtain a remote, and used it to team up with The Cartoon Hero to review the animated adaptation of Ctrl+Alt+Del. On a much more serious note, Malicia was able to track down the remote's frequency to find Chris and attack him after his review of Masked Rider.
In The Cartoon Man, Roy and Karen find a number of random objects in a hollowed-out tree, including a pen, a feather, and a glove. The pen turns out to be a "transponder" that opens a portal to an Alternate Tooniverse, and in the sequel, the feather magically turns a man into a talking piece of paper.
At one point in Worm, Doctor Mother mentions that one of Cauldron's capes can De-power parahumans. It gets used on Taylor at the end of story.
This trope was featured in Episode 3 of the TV Tropes podcast On the Tropes.