Hago's staff in The Lion King Adventures. It takes five series for Simba to use it in order to destroy the force field around the Pride Lands in The Final Task. Then it is destroyed for ever.
In Blind the Kyuubi negotiates a deal with Naruto about what he can and can't do when Naruto allows him take over of his body during the month long wait for the 3rd part of the exam before they discuss Naruto's training . This later comes in handy during the fight with Gaara and various other points in the story.
In Kyon Big Damn Hero, Nagato checks Kyon's wristwatch in one chapter. Later on, it saves his life when he gets shot, turning into an armour... skinsuit... thing.
In the 7th Sea story Homecoming, when her father Owen expresses his uneasiness about his daughter Colleen coming with the family to parley with one of the Sidhe, she just pats her guns, insisting on helping her grandmother Moira hunt down her (Moira's) hated brother. Later on, Colleen willingly shoots her father in cold blood with those same guns when she comes upon Owen trying to help his not-so-sinister-after-all uncle escape.
Manchester Lost: Angelic singing kills demons. Who knew the whole climax was going to hinge on this little bit of trivia?
In Axis Powers Hetalia fanfic Outcast Tino mentions putting a star shaped ornament on Flower-Eggs collar, it turns out it holds Torvald's crystal star.
The Fan Verse, the Travelsverse, is basically one big armoury, with Instrutilus constantly placing guns in advance and using them stories later.
Company0051, a HaloFan Webcomic: Master Chief is staying at the local military camp on Noiosi when he comes across a patch of forest with a large hole in the trees. The Captain tells him it was just a meteor. It was actually a shot-down Covenant starfighter which was carrying an AI.
Test Of Humanity, a Portal 2 fanfic in which GLaDOS implants Wheatley's brain into a human body, has a scene where Wheatley ends up overeating and gains quite a bit of weight. Towards the climax Wheatley's overweight frame ends up being what causes GLaDOS to be defeated since it causes him to be stuck in an exaust pipe blowing up a good portion of the laboratory and tearing her apart.
In the Law & Order: Special Victims Unit fanfic Girls' School, Elliot sees his reflection in the window when he is at a crime scene. He later figures out that the main perp could be identified by searching the pictures of the crime, taken by the main perp's girlfriend, for any reflections in those windows.
The original slash novel Captive Prince is notable for using a lot of those. Especially the earring comes to mind.
In the Death Note fanfic Xanatos, L contacts Matt to help him with the Kira investigation after faking his death. As it turns out, L needed Matt specifically because he was the best person to convince Mello to work with Near, being his closest friend.
In The Immortal Game, it's repeatedly mentioned that the easiest way to defeat a bladecaster is to break their faith in what they're fighting for, which makes them lose control of their blade. We see an example of this about two-thirds of the way through the story, when Sir Unimpressive defeats the Cadet, but in truth, this was setting up for how Twilight defeats Titan during the Final Battle.
In Game Theory, Miyuki finds an ice sphere created by a Jewel Seed. In chapter nine, that orb gets shattered when the TSAB attacks Nanoha and Fate's appartment, ripping a hole in reality and forcing the TSAB to close it, giving Nanoha and Fate time to escape.
Nanoha's recklessness with magic screws things up in a big way during the TSAB's assault on the Garden.
Also, Vesta's possession by a Jewel Seed turns out to be crucial to Precia's plan to revive Alicia.
The zombie animating Jewel Seed is what inspired Precia to create her temporary life-support system for Alicia to use when they travel to Alhazred. And then it turns out that Precia actually used that very Jewel Seed to revive Alicia fully.
This is discussed in the Hannah Montana story I Didn't Sign Up For This. Miley's English professor explains the trope and then Miley uses it
In the Gregg Landsman fanfic Glorious Shotgun Princess , it is used as early as chapter 4, when Kasumi Goto steals a Honey Badger from the San Diego Zoo, names it Checkov, as a direct invoking of the trope. In Chapter eleven, the honey badger turns back up as the final bit to Hock's Humiliation Conga
You Got HaruhiRolled! manages to turn the announcement of a new fanfic into one. In Chapter 56, Haruhi complains to the writer, superstarultra, about a fic he wrote in which she and all her friends are in Fat Camp. Then Chapter 81 rolls around, where the version of Haruhi from that fic appears through a dimensional portal and devours an animate cheesecake that was wreaking havoc.
In the Total Drama story, Legacy, Heather's pregnancy is mentioned thrice in the first chapter but doesn't become important until near the end.
The Powers Of Harmony: More like Chekhov's Location, but Dragon Turtle Lake — which Fluttershy and her Guards visit fairly early in the story — ends up being chosen as a meeting place by Luna after the group is scattered.
During their first fight in Episode 3, Fury Cross manages to wound Rainbow Dash with a small slash on her side, which is referred to several times over the course of the chapter as having not healed. During their rematch, it turns out to be a wrath wound, which has been feeding off of her anger and frustration at him and her friends in order to leech her strength. This allows him to nearly kill him before her friends show up to save her.
In Episode 4's teaser, Rarity collects dew pearls for a dress she's making, as it later turns out, for Fluttershy as a birthday present, which she ends up forgetting about completely due to all the time she's spending with Regal Rule. The realization of this when Sweetie Belle points it out is what triggers her My God, What Have I Done? moment.
The hoof-made wing guards Scootaloo gives Rainbow Dash as a gift just before her flying lesson in Episode 5 are later used as proof by Dash that Scootaloo can do stuff right — when everything else in that episode had convinced her she couldn't — as part of a Rousing Speech that pulls her out of her Despair Event Horizon and allows them to defeat the woebeghoul that was feeding on Scootaloo's despair.
In the Horseshoes and Hand Grenades story Month of Sundays, Santa-chan hands Akiko a camera. Later on, she and Ryu find a picture of Shotaro in his office reading a specific book, and finding said book reveals a Skull Gaia Memory and letter for Akiko to become a Kamen Rider should Shotaro or Philip (or both) were unable to help.
Literally in "Sin City Kids". Vincenzo Salvatore passes on the family revolver to Vinnie, at his 30th birthday party. Several chapers later, Billy the Kid maims Tony with the gun to avenge Vinnie.Read it here.
MLP fic A New Hero: Over the course of the first season, main character Alex gets gifts from each of the Mane Six. After Discord Mind Rapes him into such a bitter, insular person that not even Twilight's memory spell can snap him out of it, he sees those gifts, which remind him of his friendship with the others, restoring him to normal.
A new world: Story of a lost Shinobi: A picture of Naruto. Taken while Naruto was looking for Mystogan, by a guy on the train. Inadvertently starts the Running Gag of people saying Naruto is a Fairy Tail mage with him denying it. We find this out when a Rune Knight shows Naruto the picture in Sorcerer Weekly where it calls him "Fairy Tail's Orange Rocket".
Mega Man Recut has the password Protoman gives Mega Man in "Future Shock".
Dr. Light's journal is taken by Mega Man in "The Mega Man In The Moon" but doesn't come up again until the next episode.
Mega Man: Defender of the Human Race has many examples, but a specific kind of head pain is a recurring one, happening to ProtoMan upon activation, Mega Man due to the scrambler chip, and Nomad upon activation.
Chapter 1 states that robots can't self terminate at the behest of humans. This saves Drill Man in episode 8.
In episode 12, ProtoMan stealing Roll's SkyCycle led to it running out of fuel at a crucial time.
Cloud's ribbon in The Fifth Actwhich grants immunity to status effects and since it's so rare the rest of the cast is confounded to how he's immune to them. Then Cloud gives it to Kunsel when they're both captured by Hojo to help Kunsel escape. When Cloud gets possessed by Jenova, the lost of his ribbon loses him his Advantage Ball and Genesis is able to stop him.
Anthropology: Before being send back to the human worldLyra is given a necklace in the shape of her cutie mark by Princess Celestia. After Discord turns Lyra back into a pony, her magic activates an enchantment on the necklace that returns Lyra to Equestria, to she can bring the Elements of Harmony to Earth and stop Discord's reign of destruction.
Red Fire, Red Planet has a mention in chapter one that the Bird-of-Prey Hegh'QeDp is having engine trouble and had to fall out of the formation. General K'Bor complains about their cowardice, but Brokosh points out that it's hardly cowardly to shut down your reactor when when you've got a microfracture in your fusion bottle. The cloaked Bird-of-Prey is later detected by the crew of a Starfleet listening post, and after it blows up a shuttle containing two of the crew members the station raises the alarm.
Two of Jack's faulty inventions that showed up in a story of the Facing The Future Series. They came in handy to help Team Phantom storm the Guys In White's base and save Danielle.
Triptych opens with Discord granting Fluttershy the ability to permanently remove one thing. She hasn't used the ability yet, but it's been implied she's considered using it on the mysterious pony that painfully transitions between the three tribes...