Arguably, any Wide Open Sandbox will drip this trope as a simple matter of functionality- although, it might not count as you may only stumble across things after accidentally Sequence Breaking or getting lost.
Speaking of Sandbox games, the literary device gets name-checked in Saints Row IV during one mission. The Boss remarks that their next goal should be easy, to which Fun!Shaundi chides him/her for jinxing it, and CID mentions how, due to Chekhov's Gun, the next objective has to be harder now, because otherwise The Boss wouldn't have piped up about it at all.
Adventure Games are all over this. If the character adds anything to their inventory, you can almost guarantee it's going to be important for advancing the plot at some point. This depends on the game...some games won't allow you to pick up an item you won't use at some point, but others may have items that appear to be worthless because you went through the entire game without using them; but in fact you could have used that item for an alternative solution to a puzzle. For examples, see It May Help You on Your Quest.
Averted by the two Discworld games, where there really are useless things to collect, albeit not many, making them more Red Herrings.
Clock Tower 3 uses these with its "Evade Points". For example, in the second chapter of the game, almost right before you meet the Minion of the level, you can check a bottle sitting innocently on a table. Alyssa reads the label, and comments "Sounds Flammable". Shortly thereafter you meet the acid spewing Minion, you use the Evade Point located at the bottle and Hilarity Ensues.
The original Adventure Game, Colossal Cave (frequently known just as "Adventure"), subverts this: there's a room whose description goes on for pages and pages (compared to a few terse lines for other rooms), in an age when computer memory was at an extreme premium. The room has no effect whatsoever on the plot.
Space Quest 6 also does this, giving a rather detailed description for something as minor as a small alcove in the floor.
Space Quest 4 has a whole skill that proves utterly useless: the "taste" function. —>"The sewer wall tastes like... blood! You shredded your tongue!"
All of the seemingly useless items obtained for the locker at the beginning of Space Quest II come into use late in the game. The order form is placed in a mailbox to obtain the whistle used to summon the Labion Terror Beast, the Cubix Rube puzzle is used to distract said beast, and the jockstrap is used as a slingshot to take out one of Vohaul's guards.
Early on in the first game, you obtain a cartridge from the Arcada's library, which you use in the friendly aliens' underground hideout on Kerona to obtain the activation code for the Star Generator at the end of the game. Which turns out to be the last four digits of Sierra On-Line's phone number.
In Crysis, during a late-game lull in the action, a technician conspicuously introduces an experimental gun that fires guided nuclear missiles. Your character asks (half-seriously) if he can try it out, and is unsurprisingly denied; you end up retrieving it later after everything goes to hell, and it is instrumental in defeating the final boss.
In Dangan Ronpa, there is a sixteenth seat in the courtroom. When the time comes, there is also a sixteenth student to take the seat.
It's pointed out early on that the door to Naegi's bathroom is stuck and needs to be opened a certain way. This then becomes a key piece of evidence once the first investigation comes around.
When a new floor is opened up, the game makes the player take a tour around every room and examine certain things, thus making everything they examine a potential Chekhov's Gun for the next murder.
Inverted in Devil May Cry 3, where human-sized chess pieces are seen in Mission 4 and must be destroyed to pass an area. 3 missions later, it becomes apparent that these chess pieces, now animated, form a type of enemy. Not something helpful.
Played straight in the first game, with the biplane "Carnival". It does nothing in the first mission, suspended by strings (along with some marionettes that are definitely enemies), but at the end when the player is meant to think that Dante is screwed, it crashes through the ceiling in perfect working condition just waiting to be used.
In the adventure game Blazing Dragons, one of the items you start with is Flicker's "clicker". Every time you look at it, Flicker says that it was his first ever invention, "the practical use of which escapes (him) at the moment". Its only use is in the very last scene of the game.
The princess's lute in Final Fantasy I, acquired after the first quest and necessary to complete the last.
In Final Fantasy IV, there is a quest where Yang's wife asks you to find him in the Dwarf's underworld Kingdom. Once you find him and go back to tell her you did, she gives you a silver kitchen knife. A horribly unpowerful knife that is useless in normal combat. Useless until you get to the part where you have to defeat Odin to get him as a summon. This little otherwise useless knife deals 9999 points of damage to Odin, enabling you to defeat him in one "round", way before he has time to unleash his all powerful attack "Zantetsuken".
Aeris Gainsborough owns a materia in Final Fantasy VII that seems to be of no use at all, but proves later to be the materia that summons the ultimate defensive spell, Holy.
Additionally, during the Shinra building raid, Cloud finds a megaphone in a locker and decides not to take it. It turns out to be belonging to Reeve - better known as Cait Sith, who appears as a character later using a megaphone as a weapon. You can later return to this locker and pick it up - it's Cait Sith's ultimate weapon.
A literal example occurs in Junon: you see the giant cannon the city is built around on Disc 1; it gets fired on Disc 2.
In Final Fantasy: The 4 Heroes of Light, a sign near the windmill in Horne explains how to properly treat sheep. In the next-to-last dungeon, these instructions are key to a puzzle.
The battle scene between Greil and the Black Knight in Fire Emblem: Path of Radiance sets up two. First, the Black Knight gives Greil a sword to fight with, which is revealed to be Ragnell, the only blade capable of opposing the Black Knight's Alondite, later in the game. Greil, who chose to give up the sword, turns it down, saying in regards to his axe, "The only weapon I need...is right here!" Just before the final chapter of the sequel, Radiant Dawn, the axe Greil was wielding returns—it's Urvan, an absolute Game Breaker as it is not only the most powerful axe, it also has an accuracy of 110, brutally subverting the "powerful but innacurate" nature of axes in general.
Except for those damn copyright protection sequences. AARRRGGH!
This is somewhat negated in the PC CD-ROM adaptation, though.
In the Ace Attorney games, seemingly random, irrelevant things such as a metal detector or a picture of the police mascot are often inexplicably added to the Court Record as evidence. They will later prove to be crucial in cracking the case. Interestingly, the logical usefulness of an item is almost always inversely proportional to its actual usefulness: murder weapons, security camera videos, and photographs sometimes border on useless, while clay fragments, picket fences, air tanks and guitar picks are crucial to solving the case.
The main idea is that the consequential evidence is clearly pointing the guilt of the defendant, and Phoenix's only weapon is to point out the inconsistencies and lies in their testimonies with the seemingly innocuous evidence, as those are the details that everyone overlooks. Phoenix only uses the consequential evidence when he's managed to provoke testimony in conflict with it.
The best example might be in the last case of the first game. Very early in the case, you acquire a screwdriver that was evidence in a completely separate case. According to Miles Edgeworth (whose car the victim was found in), it has no relevance to the current case at all. Later, when you need to prove how the killer was able to place the victim's body in Edgeworth's car, it's revealed that when Edgeworth was summoned by the killer to the police station to transfer the screwdriver to his office, and it was at the station that the body was placed in his car's trunk. Phoenix himself remarks that it was the most useless piece of evidence in the entire case up until this point.
This can be seen as justified in a series such at this one however. Unlike a hero saving the world, criminal court cases do often require you to have useless documents, information and evidence just because it exists, with you because a hero saving the world would need all the space he can get for important stuff, while in a criminal case, EVERY item can be considered important because you never know when a useless piece of evidence is going to suddenly become useful. Thus, this concept of "keep every item, document, and piece of evidence you find" is actually much more realistic then it seems at first.
In The Legend of Zelda: A Link to the Past, nearly every boss has a piece of equipment that Link can discover to use against it. In fact, The Dragon, Aghanim, can be brought low by the humble Butterfly Net, one of the earliest pieces of equipment found.
For that matter, in OoT, Ganodorf's magic can be deflected by the common bottle.
That may be, but the bottle is already fairly important (Link can use it to house fairies and lon lon milk that can restore HP in a pinch) so it isn't exactly an example of Chekhov's Gun.
And then in The Legend of Zelda: Twilight Princess, the very first item Link receives is the fishing pole. Seems relatively pointless, other than for...well...fishing. Turns out that, like the net from LttP, it's a weak point for Big Bad Ganondorf in the final battle; you can't hit him with it, but you can distract him while you get in a few good shots.
Oh, goody, a new and really good one. In The Legend of Zelda: Skyward Sword, there is a statue of a bird's head in Skyloft, that appears to be missing an eye. Odds are, you walked right by it without even noticing, or you may have noticed, but not really paid much attention to it, while on the way to free your Loftwing at the beginning of the game. Well, that missing eye is acquired late in the game, and is actually the key to unlocking the statue's secret: it activates a cannon in the head of the bird statue that opens up the last dungeon of the game.
In Ratchet & Clank Future: Tools of Destruction, The Plumber gives the heroes at one point a "3 3/4 centicubit hexagonal washer" "just in case". This item is utterly worthless through the game until the final cutscene, where they use it to fix the Dimensionator before a massive black hole swallows them up.
Around the same time, you have King Dedede going around, seemingly a villain, "trophy-fying" heroes and taking them, seemingly on the same villainous side as Wario. Until he robs him. Then it seems that Dedede just wants to have his own private collection of trophies of the heroes, complete with dressing them up with odd badges, screwing around with the mission at hand (and something Dedede, at his most annoying, would plausibly have done). Until, way at the end it turns out the badges Dedede put on them were time release resurrectors, and it was Dedede's plan all along to, in case the heroes failed, save them with his own backup squad. It works, very, very well.
This is pretty par for the course for Dedede. He's generally found engaged in activities that are taken as villainous or at least troublesome around the start. Come the ending it turns out that, on such occasions as he wasn't acting in the best interests of everyone at large, he's being mind-controlled by the real villain. He's a bit of a Chekhov's Gunman like that. I think I'm using that right, anyway...
The badges Dedede uses have an odd twist on this. When they're first introduced, it's completely understood that they're going to be Chekhov's Gun, and instead the player wonders what part they'll play instead. This is also built up with a scene that Kirby picks up one of them, ponders as to what it is, then runs off. Sure enough, they turn out to be character resurrectors and are key to the plot. However, what's always missed is that Kirby had the one that was presumed to be build-up for the other characters, meaning that he was resurrected too. Quite sneaky!
Oh, and also inverted, in that the one Kirby found was Dedede's own one, shown by how Dedede didn't have one for himself, meaning he didn't get resurrected. But it turns out that the ones he put the badges on resurrected him anyway, so it didn't matter.
Resident Evil 5 manages to use this. Sheva picks up a syringe of serum from an attaché case. Both Sheva and Chris learn that the serum is actually one used to keep their enemy's superhuman powers in check, and that too much of a dose can hurt him.
In Fate/stay night Unlimited Blade Works scenario, before his death, Lancer activates his Ansuz Rune to burn his dying place for almost no apparent reasons. That action indirectly saved Shiro, Rin and Saber from a fatal, unprepared encounter with Gilgamesh, which forces him to retreat.
It also inverts this trope in another instance where the "gun" in question becomes important after its used. In the prologue, Rin uses a literal family jewel to save Shiro's life and leaves it with his body. Archer returns it to her later that night. Yet in Unlimited Blade Works and Heavens Feel, Shiro is revealed to have the same jewel, and when Rin sees it she realizes The Reveal, Archer is who Shiro will become, and he returned the jewel after carrying it for his entire life (and then some) until he saw her again.
5 Days a Stranger has a perfectly textbook example (perhaps intentionally): one of the very first rooms the player enters has a big shotgun hanging up (yes, over the mantelpiece) but you can't walk off with it, because Trilby refuses to lug a big heavy gun around everywhere. The final scene of the game takes place in that room, and the gun is used to solve the final puzzle.
In Earthworm Jim, you launch a cow into the sky at the very beginning of the game. At the game's end, when you defeat the Queen, the cow comes flying down and crashes onto the princess you just saved.
In Xenogears, right at the start you can buy an accessory that prevents fuel drain. This is apparently worthless, since nobody in the game * has* a fuel drain attack... until you reach the final boss battle, where fuel drain can become a crippling problem if you're not careful. (No other stores after this one sell the anti-fuel drain accessory.)
An available accessory that appears to be worthless, with no indication that you should buy it but without which the final boss battle is unnecessarily hard? That's not an example of this, that's Guide Dang It in action
What about the Mermaid tear, which you get at the very beginning of the game, and which you can't use until during a certain end-game side-quest?
In Super Robot Wars, Lamia Loveless (also her distant sister Aschen Brodel) is installed with Code: DTD, which serves as a 'memory reboot', that even her creator Lemon Browning deems "You probably won't need it in this war...". But then in OG Gaiden, it serves to be a truly important device when Axel Almer saved her from Duminuss and ODE influence, by resetting to the point that their alteration never occurred. Her distant sister Aschen from Mugen no Frontier, however, uses it on regular basis to kick the enemy's ass.
In Saisei-hen, Zero accidentally placed a geass on Esther after she turns into a Dimentional Beast, commanding her to stop when she attacked the Shinkirou. Few chapters later, Crowe believes that Esther has completely turned into a DAMON, to which Marguerite tells him otherwise. DAMON Esther attacks him at that moment, only to halt in its tracks after Zero reissues the same command, proving to Crowe that Esther isn't completely a DAMON yet, and still retains her humanity to a certain degree.
In Metal Gear Solid, in every game including the two prequels,the pack of cigarettes is highlighted early on in the game- later, Snake can, and indeed, must, use these to detect security lasers. There are other uses for them.
There's at least one other way to detect the lasers, so he doesn't have to use the cigarette.
The USS Missouri in MGS4 fits this trope. Insignificantly introduced early in the game as a real-life WW2 battleship that had been recommissioned as a training vessel, it later becomes the only ship in the US Navy to survive the Big Bad's plan to disable all the weapons in the world(due to not being linked to the "System" which controls all of them), and ends up carrying and supporting the main character in their assault on his floating fortress.
Another good one: as Snake is about to shoot himself in front of Big Boss' grave in Guns of the Patriots, he notices that there is a flower bouquet in front of the grave right beside the one he's in front of. It makes sense after the fake credits when you realize that said grave marks the resting place of the Boss, Big Boss' spiritual mother. Now guess who dropped the bouquet before Snake came.
In the prologue of Metal Gear Solid 2, which centers on a US Marine Corps. Metal Gear model, a brief mention is made of a Metal Gear project led by the US Navy. Much later in the plot, the Navy's Metal Gear makes an appearance and turns out to be a significant part of the plot.
From the beginning of Metal Gear Solid 3, Naked Snake has a mask in his inventory which seems to serve no real purpose. Speaking to Major Zero reveals that "its creator pitched a fit" when the mission it was intended for was scrapped, and that Snake was given the mask (instead of it being thrown away) because it might come in useful. Later in the game, the mask becomes an integral part of infiltrating Groznyj Grad to rescue Sokolov, as Snake has to neutralize and disguise himself as Major Ivan Raikov. Coincidentally, the mask was originally made for a mission wherein a CIA operative would have to disguise themselves as the same man, thus it bears an uncanny resemblance to Raikov. So uncanny that even though the disguise fails, it fails because the mask is so damn convincing that Raikov's lover, Colonel Volgin is convinced by it and only realizes his mistake after he grabs Snake's genitals. Twice.
The same mask can also be used during the fight with Colonel Volgin to trick him into thinking that Snake is Raikov, which throws him off guard and allows Snake to get a few hits in. After tricking Volgin this way, his attacks become more frequent and damaging.
In Halo 3, 343 Guilty Spark has a special eye laser that he uses to fight off Flood and kill Sergeant Johnson.
There's a Chekhov's Gun earlier in the series. In the first Halo, Cortana steals Installation 04's Index, the only known way to fire the Halo. At the end of Halo 3, Cortana uses that same Index to fire the new Halo and wipe out the Flood.
Very few people then remembered that the title of the sub-level where you retrieve the Index is called 'The Gun Pointed At The Head Of The Universe.' This is a literal Chekhov's Gun.
An interesting one for Halo 3 ODST. In the ViDoc Desperate Measures Buck mentions that they "can even commandeer the city's garbage trucks if we need 'em." Well guess what you wind up protecting in the last level?
How about that Nova Bomb in First Strike that showed up again in Ghosts of Onyx. Not in the games, but close enough.
Mass Effect. Throughout the Citadel there are these innocuous insect creatures called the Keepers, who don't talk to anyone and only seem to exist to keep the impossibly ancient space station running. It turns out that the Keepers' job is to maintain the Citadel because it is a giant Mass Relay that will bring the Reapers into the galaxy. The Keepers' job is to enable civilizations that discover the Citadel to use it without realizing the stations' intent, enabling the Reapers to hit the center of galactic civilization first and without warning.
Mass Effect is replete with these. Who would guess that the Mass Relay sculpture in the Presidium was the destination point of the Conduit? (Though if you have Kaidan in your group he'll comment that the statue is making his teeth vibrate.) Or that the krogan genophage and the Rachni War would become important plot points on two of the planets you visit later?
A very subtle example takes place in the Citadel Council tower, if you have Ashley in your party. She'll comment that "I bet these stairs aren't just for show. They'd make for good defensive positions if this place is ever attacked...." Turns out, you are the one who does the attacking at the endgame.
A very minor example occurs if you choose all the paragon interactions with the Asari Consort. She gives you a seemingly worthless trinket that you can later use on another planet to unlock a cache of valuable equipment.
Double Subverted with the Great Rift on Klendagon, a geological feature prominently noted as being caused by a miss from a powerful weapon. Come the second game it turns out the important thing wasn't the weapon itself (it's millions of years old and inoperable), but what it was shooting at (a now-dead Reaper to go poke at for answers).
The Leviathan of Dis which is mentioned in a planetary description in Mass Effect 1 is revealed in the third game to have been a dead Reaper that the Batarian Hegemony picked up to research. But dead Reapers can still indoctrinate and it indoctrinated their scientists and politicians which left the batarians defenseless when the Reaper invasion finally came causing them to be steamrolled over.
It's worse than that. The Reaper corpse turned out to be comparatively unimportant to the REAL Leviathan of Dis, the thing that killed the Reaper. Specifically, the thing that many people assumed was just a Shout-Out to Farscape led to a chain of evidence that culminated with the discovery of the Leviathans, the Abusive Precursors to the Abusive Precursors and the things the Reapers are based on. They're still alive and as pissy as ever.
The most hilarious example of this in the game is Samantha Traynor's toothbrush. She mentions it in one of possible dialogues pretty early in Mass Effect 3, saying that it uses mass effect fields to break up plaque and massage the gums. You later use it in the Citadel DLC to infiltrate the stolen Normandy. Worth every one of the 6000 credits she paid for it!
Shepard: If you told me this morning that a toothbrush would save the Normandy, I would've been very sceptical.
In the original Mass Effect, the flavor text for a backwater nothing planet called Klencory indicates that a mad Volus has claimed the planet due to a vision of a "beings of light" who were to protect the universe from "machine devils." In 3, The Catalyst turns out to be a small hologram who gives Shepard the options to destroy, control, or achieve synthesis with the Reapers.
A more darker example would be the knife strapped to Tali's boot since her appearance in Mass Effect 2. She uses it to kill Legion on Rannoch in the third game if you botch negotiations between the Geth and Quarians badly enough.
Front Mission: Gun Hazard winds up giving us a Chekhov's Laser Platform by way of a solar energy collector subcontracted out to The Syndicate.
In Taiyou No Shinden Asteka II (a.k.a. Tombs and Treasure), you get the lighter from the first room in the game, and it can't be used for anything until the last room in the game, where it's necessary to complete the game. You obtain a silver key at the same time as the lighter, which is later used to unlock the Temple of the Sun and acquire the game's prime MacGuffin, the Sun Key.
The World Ends with You. In the second chapter, in the cutscene before the second to last boss, Megumi reveals the only thing protecting Neku from his brainwashing is his player pin, so he imobilizes Neku and crushes the pin. It didn't work. Why? Because in waaaaay back in Chapter 2 of the first week, Shiki points out how Neku has 2 player pins, the extra given to him by Josh.
In Loom, the first spell cast in the game (and that is periodically replayed to you through it) is the last spell you cast.
One of the first things the Postal Dude's Bitch says in Postal 2 (before the game actually starts) is "don't forget my rocky road." At the end of Friday (the last day), she nags the Postal Dude about her rocky road again (after not being mentioned throughout the rest of the game), to which the Postal Dude realizes that he completely forgot about it from the very beginning and shoots himself in the head to escape his wife's nagging. This leads to the events in the add-on, Apocalypse Weekend.
Mega Manloves this. It's a safe bet that the most bloody useless Robot Master weapon you get will be the one Wily's weak to. Most extreme offenders: Mega Man 2(Bubble Lead), Mega Man 3 (Top Spin, except it is quite useful if you know how to use it just watch ) and Mega Man 7 (Wild Coil).
Used and reemphasized to the point of deliberate annoyance in Space Quest VI. "Hey, you forgot your fish!"
Hotel Dusk: Room 215 features multiple subversions. First, the inconspicuous sewing machine and adhesive remover that come in a package near the beginning, and whose only introduction is Kyle commenting that they're useless prove to be essential to completing the game. On the other hand, the screwdriver that he repeatedly and visibly proclaims will surely be useful... has absolutely no possible use at any point in the game — the only thing it can accomplish is getting you a Game Over if you don't put it back at the right time.
Also, in one alternating cut scene in the snowy town I forget how to spell Flanoir, depending on who you talk with as Lloyd, you get a different trinket. Later, the trinket saves Lloyd's life by keeping an arrow from piercing his chest.
Actually, the only characters who will give you a trinket are Zelos, Colette or Kratos. If you pick anyone else to talk to in the Flanoir scene, Lloyd will just dodge the arrow.
The Fire Spears from the first Suikoden. When first introduced, it seems to serve no purpose, but Odessa insisted that Someday This Will Come in Handy. Later, the Liberation Army is flawlessly beaten by Teo's Armored Cavalry... only after getting back the Fire Spears they end up winning.
The Fire Spears also come back in the sequel, where they're first used to defend the Mercenary Fort against the Highland Army, though they lose effect in the next battle. Shortly after the player loses the second battle, the Fire Spears are again used to distract Luca Blight while everyone escapes.
Possibly a case in Sonic Adventure. Immediately after the first level (Emerald Coast), Tails explains to Sonic that his new Tornado II prototype is powered by a Chaos Emerald. Near the end of Sonic's story, the monster Chaos gets six of the seven emeralds. Where was the seventh? Still inside the Tornado II. May not directly count for two reasons: 1, the emerald in the airplane is different than the one referenced earlier, and 2, the Chaos Emeralds were established from the very beginning (arguably from previous games) as being supremely important.
When replaying Silent Hill, you find a device in the 7-11 lookalike that is of no use unless you're at certain locations (e.g. the rooftop of the oxidised Midwich Elementary) through which you get the Alien ending and a raygun for the next replay.
In Silent Hill 3, you have Heather's pendant in your inventory from the start. There's no indication that it's important and all you see when examining it is a little red bead-like thing inside. This turns out to be the one thing you need at the end of the game. It's actually the Aglaophotis in pill form. If you use it when Heather is about to birth God, it will cause her to throw up the fetus. Claudia eventually swallows it in an attempt to save it and dies a horrible death.
Same for the vials of Aglaophotis in the first game, one of which you use to exorcise the Puppeteer Parasite from Cybil, the other of which Dr. Kaufmann uses to separate Alessa and the Incubus.
Also in the first game is the Flauros, which Harry finds immediately after meeting Dahlia for the first time. In the amusement park towards the end of the game, Dahlia uses it to break into Alessa's otherworld and stop her from sealing the God.
In The Secret of Monkey Island, the pirate drink "grog" is referenced early on, and a pirate in a bar says the stuff is so strong that it can "eat through a pewter mug". It's also described as "the most caustic, volatile substance known to man!" Later, you must use grog to eat through the bars of a prison ... and you have to use several pewter mugs to transport it there as it keeps eating through them!
In another example, Guybrush Threepwood comments early on in the first game that he can hold his breath for ten minutes, a skill he considers useless. It ends up working wonders later in the game when he's thrown off a pier with an weight tied to his waist.
The writers were so attentive, that if you wait 10 minutes while Guybrush is underwater, he will actually drown right at the 10 minute mark. It is the only way to lose the game.
At one point you can pick up a staple remover, which Guybrush remarks will probably come in handy. Beyond its initial use in a cutscene where it is used on a Yak, it is useless throughout the rest of the game. You have the option, however, of throwing it in the cooking pot on the ship, which is also a way to lose most of the items that could come in handy later. However, if you choose not to do this, you will lose it before you get to Monkey Island anyway, as the original game removed the items not needed as a memory saving measure.
In Monkey Island 2: Le Chuck's Revenge, you can you buy some clearly irrelevant items in the shop on Booty Island (such as the Elvis collector plates and hub caps), they come to no use in the game, and you can run yourself broke by buying them. With no other way of earning the money, this can mean you are effectively stuck in the game. Also, you can take out all the books in the library on Phatt Island, but only three of them are useful in game. Most of the other books will provide generic responses such as "This isn't that interesting, I'm not sure why I checked it out", though some, such as those written by Elaine (under her pseudonym Melanie Leary) will provide unique responses (they're mostly about how much she disliked Guybrush when she first met him).
The Monkey Island series often plays this straight, (it is an adventure game, after all). It almost always subverts it as well. While most items you pick up must or can be used at one point or another, there are always a couple items you can pick up (usually towards the beginning of the game) that have no use whatsoever except for humor value or extra background flavor.
Possibly the best use of this trope is when Stan hands you a bunch of random advertising pamphlets, seemingly with no use whatsoever. However, one of them just happens to be called "How to Get Ahead in Navigating". And when you encounter a group of people looking for a navigating head...
But then subverted in The Curse of Monkey Island, where Bloodnose the Pirate gives you similar pamphlets that turn out to do absolutely nothing.
A particular heartwarming example from Tales of Monkey Island: You receive Elaine's wedding ring in Chapter 2 and keep it throughout all of the episodes, but Guybrush refuses to use it for anything. It's the solution to the last puzzle in the game.
Done in Castlevania: Dawn of Sorrow; at one point, Soma is given a good luck charm from Mina Hakuba, his not-girlfriend. If you don't equip this item before going into a certain cutscene, Soma falls for the trap set by the villain and becomes Dracula. It's also done later on to get even further in the game; beating the boss Paranoia gets you the ability to enter mirrors and use them as portals. When you finally reach the pinnacle of the castle where Dario is waiting, you notice a demon lurking in the mirror behind him, boosting his power. Entering the mirror triggers the real boss fight with Aguni. Beating Dario just ends the game prematurely.
Subverted in Left 4 Dead. In the No Mercy campaign, there was an incident where the pilot who's gonna save you says there was an incident that happened. If you went through the commentaries, you would know that originally, it would be revealed he picked up an infected person who bit him, which caused the helicopter to crash after he turned as well. This was scrapped because, as playtests showed, people felt that a sense of accomplishment was taken away from them by that scene, so they just got rid of that ending bit, rather than fix it.
This is actually soon to be un-subverted. Valve has just announced a new campaign that takes place after No Mercy where the helicopter crashes called "Crash Course."
Re-subverted in The Sacrifice comic. The comic reveals that the survivors are immune carriers of the disease, which doesn't infect them, but infects all non-immune with which they come in contact, including the helicopter pilots.
Disgaea: Hour of Darkness uses Flonne's pendant in this fashion. One, it's an indicator of Laharl's Character Development (it burns hotter than the magma he fished it out of when it's introduced but does nothing when he grabs it near the end). Second, it's a sneak peek at the motives of two other characters who touch it - Dark Adonis Vyers (aka Mid-Boss aka benevolent Overlord King Krichevskoy) and Vulcanus (whose intentions are just as evil as he looks).
The four leaf clover seal on Rozalin in the second game. Turns out to be a seal on the real Overlord Zenon, a cosmic level overlord.
The Nancy Drew games are in love with this trope. There are many instances where the player will come across something that appears to be useless until the end of the game, which include:
The fire alarm in the second game, which will guarantee a game over if pulled too early, but will save Nancy's life at the end of the game,
The chandelier in the third game, which once again guarantees a game over if untied too early, but is used to trap the culprit at the end, and
A ring won on the carousel in the eighth game that becomes useful not once, but twice at the end. And these are just a few examples...
Subverted by Uninvited. There are three rifles hanging on the wall... but there's no key to open the gun rack.
Lampshaded in Discworld Noir, when Lewton notices a grappling hook behind the troll he's trying to question. Sure enough, while he can't collect it immediately, he gets to use it later. "I couldn't have been more interested if it had had 'Plot Device' written all over it."
The first day of the first route of Tsukihime has Shiki bringing an unidentified white ribbon with him for no particular reason. When you eventually get to the maid's routes, it's revealed this is a keepsake Kohaku gave as a sort of promise for him to come back and give it back to her. The importance he places on it, whether or not he remembers and also identifies whose it is becomes very important. As in, Akiha and Kohaku can die if he thinks Hisui gave it to him. Not bad for an item mentioned in one sentence offhandedly when Shiki is unpacking, eh?
Semi-subverted in the first Broken Sword. At the start of the game George is shocked with a hand buzzer by the owner of a joke shop, who then laughs and gives it to him as a gift. When George tries to pull this prank on anyone else in the game, they all refuse to shake his hand for one reason or another. It's only when an assassin has George helpless at gunpoint at the top of a mountain that George gets a chance to use it, shocking the assassin and dramatically leaping to safety while he fumbles with his gun.
Most of the early text based adventure games (e.g. Adventure and the Zorks) had you controlling a character traveling through what was essentially a maze of rooms in which were occasionally placed certain things that you would use later; i.e. "You're in a small room with exits to the east and the north. You see a small table here. You see a flashlight here." You could generally plan on needing that flashlight later so you would, "get flashlight".
Interestingly, the sequel goes back to the traditional tactic of not only having every single item be used at least once, but if the item is small enough to be passed through time, it will be needed in another time.
The only item that's never used is the hubcap....and you can not pick it up.
Standard policy for adventure games is that if it's not nailed down, take it, you'll need it. If it IS nailed down, find a way to remove the nails and take it. And take the nails too. Many, many early adventure games punished people for following this advice before realizing that it was a bad idea. For example, in Uninvited, picking up a certain seemingly important gem results in being demonically possessed about three turns later. Whoops.
Many adventure and RPG games condition pack-ratting behavior as an inventory management pressure, especially if there are inventory limitations and/or economic necessities. Not all games give clues whether the items are useful for problem-solving, or at least for uncovering Easter Eggs, or just Vendor Trash or completely dead weight. Recently the games have gotten easier by simply making the 'Handy' things undroppable/unsaleable, rather than more intuitive in their problem-solving application.
Mana Khemia Alchemists Of Alrevis introduces us to Sulpher, Snarky Nonhuman SidekickMana of The Hero Vayne. Sulpher knows a lot more of what's going on than what he's been letting on, and he is quite strange for any ordinary Mana, and that's saying something. As it turns out, Sulpher is not a Mana. He's just an ordinary house cat. Vayne, on the other hand...
In Fallout 3, when you first enter the Citadel laboratory you see a giant robot that some scientists are working on. At the end of the game they finally get it to work and help you in the assault on the memorial.
The code for activating the machine at the end of the game, saving the wasteland and sacrificing your life, turns out to be the numbers of your mother's favourite verse. Your father mentions this conspicuously so far back that you're an infant at the time.
In Fallout: New Vegas, you find "Euclid's C Finder" early on in the game. It appears to be a child's toy and does nothing other than act like a laser pointer. After completing a certain mission a certain way, you discover that this "toy" happens to be the aiming device for the Archimedes system. Once per day, you can trigger a devastating orbital energy weapon that obliterates pretty much everything in a wide circle around the area that you point it at.
Also, the Platinum Chip that Benny stole from the Courier at the beginning is the key to upgrading Mr. House's Securitrons.
At the beginning of the Call of Juarez: Bound in Blood, Ray and Thomas kill a company of Union troops attacking their family estate. Later, Colonel Barnesby and his men come by and collect all the rifles off the dead troops. These rifles become a major MacGuffin later in the game's main plot.
Jak and Daxter: The Seal of Mar at first seems like it's just a way to identify the Kid as the lost heir... until we find out that it seems to possess some mystical properties, and it confirms that Jak is Damas's son.
In Bioshock, when the sub carrying Atlas' family is blown up, there's no sign of any bodies from it — a bit odd, given the game's attention to detail, but it might be an oversight or they just felt they weren't necessary. It later turns out Atlas is Frank Fontaine, and made the family up. That's why there were no bodies. In Sander Cohen's level, you can find posters for a play called Patrick and Moira - the names "Atlas" give his made-up son and wife.
Also in Bioshock, there is an out-of-the-way audio log that mentions that the genetic locks for bathospheres, which the player has been using since the beginning of the game, are inaccurate, and that anyone closely related to someone with access would be able to use them whenever they want. This is an early hint at the player being Andrew Ryan's son.
Early on in Star Ocean: The Last Hope, Reimi scolds Lymle for drawing on the floor of your spaceship. It is played for laughs in a "precocious child" sort of way. The drawings actually form a gigantic protection symbol, which turns out to be the thing that ultimately saves the Calnus (and everyone onboard) from being completely destroyed during the assault on Nox Obscura.
In Star Ocean: The Second Story: It's introduced early in the plot that Rena had a pendant with her, as the last memento of her old family. Well as it turns out that pendant acts as a key to bring about Expel's Armageddon several hundred years before it naturally would.. Who would have thought?
Koudelka, less-well-known prequel to the Shadow Hearts franchise, has the main character lose something in the opening FMV. You can find it again about 3/4 of the way through the game. Then you have to wait for the pendant to actually be useful, which isn't until a pre-final-boss cut-scene. Didn't pick it up? Instant game over.
In Modern Warfare 2, as Price is giving his inspiring speech, a knife appears on the screen when he talks about killing the Big Bad. Guess what weapon kills the Big Bad...
And while we're on the subject of Call of Duty, in Black Ops, at the end of the first mission of the game, a cargo ship is briefly visible. It's later revealed near the end of the game that it's the ship where the numbers broadcast is being transmitted.
During Modern Warfare 2's museum level, there are two mysterious soldiers in Juggernaut armor which are the toughest enemies you can fight in the game. Guess what Price and Yuri wear during Modern Warfare 3's final mission...
In Chrono Trigger, at the beginning of the game, Crono bumps into Marle, knocking her pendant off, and giving it back to her to have her join you. She refuses to sell it because it has "a lot of sentimental value". Later, we find out the Pendant is the same as Schala's pendant, and is powered by Lavos' energy to open up the mysterious boxes and doors you found littered throughout the game, 'til now.
In the first Mata Nui Online Game, after the Po-Koro event, as a reward for helping the town, you are given an item, the "Po-Koro chisel" which seems to have absolutely no use, surprising in a game where every single item serves at least some purpose in one way or another. Flash-forward to the ending cutscenes of the game where Takua is fleeing from the newly-awakened Bohrok, and he discovers a device with an indentation that bears a staggering resemblance to the chisel. If you can't guess what happens next, you haven't been paying much attention to this page.
Dragon Quest VIII: After helping Prince Charmles collect an Argon Heart for his Rite of Passage, the Royal Brat promptly renders the whole exercise pointless by buying a bigger heart, leaving Eight and his friends with a pretty but pointless trinket. However, the heart comes back into play towards the climax, when the King of Argonia reveals to Charmles that he saw him buy the heart, and kept silent as a Secret Test of Character that Charmles failed with flying colors. Then the good ending reveals the Heart's true purpose: with Eight's Secret Legacy revealed, the Argon Heart now stands as proof that he finished the Initiation and is worthy to rule instead of his cousin Charmles.
In Crash Of The Titans, in the opening cutscene, Coco tries to get Crash to help her get a butter-recycler working. She asks him to hand her the 'Transpoolooper', a purple spanner thing. they are then inturrupted by Cortex in his big blimp, setting the plot in motion, and Crash puts the Transpoolooper in his back pocket. At the end of the game, they need to stop the giant Doominator robot. Cortex claims that it can't be stopped so easily, and Coco counters that she "could do it in seconds if I had my Transpoolooper"...and Crash pulls the required tool out of his pocket.
In EarthBound, the Meteor that starts off the adventure by bringing Buzz Buzz to Onett is used much, much later to gain the material used to go back in time to defeat Giygas.
In Mother 3, the Courage Badge you are given early on is revealed later to be the/a Franklin Badgenote it's possibly the same one from Earthbound and Mother, which makes it a potential series-wide gun and is crucial to the final battle.
Still in Mother 3, the new year's eve bomb that you can buy from an armament shop is an expensive one-use weapon that has no effect whatsoever when it's used. Until you run into the Porky's Statue that the bomb is the only way to defeat, dealing 99999 damage (hint: the statue has a million HP, wich is insane for the game, and will most likely kill even a fully leveled party in a few hits.
In Pokemon XDDr. Kaminko says that his Robo-Kyogre will never be of use after you manage to defeat Robo-Groudon. It later turns out to be one of the only inventions of his that is; it's one of the few vehicles able to get past the artificially induced currents and storms surrounding Citadark Isle.
In Pokémon Black and White in the museum in Nacrene City just before the gym, there is a Dragonite skull, a fossil, a meteor, and a regular rock found in the desert that is only there because it looks pretty. It turns out that said rock is actually the Light/Dark stone that contains the spirit of Reshiram or Zekrom depending on your version.
Happens in Mace Griffin Bounty Hunter. Midway through the game, you find a strange gun which has the unique property of having unlimited ammo but is pathetically weak against normal enemies, making it mostly useless... up until the final level, where it turns out it is the only weapon that can harm the Watchers aliens.
Dragon Age II. In the end, the driving force behind the entire plot turns out to be Bartrand's Lyrium Idol, a seemingly minor Artifact of Doom that briefly appeared towards the end of Act 1.
The qunari, who know how to make gunpowder, show up in Act I... but no one else successfully acquires this knowledge until the infamous "boom scene" of Act III.
A seemingly innocuous Fetch Quest turns out to involve gathering the ingredients for the bomb used in said "boom scene."
In Portal 2, a Cave Johnson recording mentions that moon rocks are highly amenable to portals. In the final boss battle, guess how you dispose of the end boss?
It's foreshadowed earlier than that. One of Rattman's dens in the earlier test chambers shows a rough sketch of a cat jumping over a moon.
And even earlier and subtler that that. A radio found off of one of the test chambers plays a seemingly pointless series of noises. If you run them through an MMSSTV program, it reveals a picture of a companion cube on the moon.
At the end of World 1 of Super Mario Galaxy 2, Bowser Jr. can actually be seen piloting a small spaceship shaped like an evil smiling head. That spaceship is later revealed at the end of World 3 to be the head of Megahammer, the level's boss. Once Megahammer if finally defeated, its body can be seen one last time as the first planet encountered in the final Bowser Jr. level.
Later on, you run into several Green Lumas who tell you about "120 cosmic jewels."
At the very beginning of Golden Sun, The Wise One does... something just before Isaac and Garet leave Sol Sanctum with the Mars Star. At the very end of the sequel, it's revealed that at that time, the Wise One imbued the Mars Star with a fraction of the Golden Sun's power, which seeped into Isaac over time. The end result of this is that when Alex bathed in the light of the Golden Sun, he got slightly less power than he expected. He was slightly less than omnipotent and slightly less than immortal, which allowed the Wise One (who is omnipotent and immortal) to shut him down and trap him on the collapsing Mount Aleph before Alex could make good on his evil plans.
A subverted one from the first game: Entering the areas Tret has cursed causes an instinctive force-field Psynergy to protect the kids from the curse. Garet comments that it would be neat if they could learn to use it consciously. It never comes up again.
And in Dark Dawn we have a rather spectacular one. The effective Noob Cave of the game is a supernaturally darkened forest. Isaac comments that you should light up these places to drive creatures of darkness away... with Fireball, for instance. The second half of the game involves an artificial/supernatural Total Eclipse of the Plot covering about half of Angara, and you have to find and turn on a light big enough to dispel it.
The Orion Conspiracy has every item you pick up as this. Yes, every single item you pick will have a use in one form or another. In the case of some of the items, it is not too difficult to figure out what to use them on. For other items, it will be difficult to figure out what to use them on.
The Reconstruction has two, one of which is an actual object and one of which is a seemingly-throwaway factoid. The first is the 'artifact' Havan finds in the mine, which turns out to be an Immortality Inducer. It is also the reason why Fell brushes him aside in favor of Six Stars, due to his obsession with it. This inevitably results in Havan going on a Rage Against the HeavensFace-Heel Turn and throwing the entire plot Off the Rails. The second one is the fact that Havan technically becomes the "leader" of the si'shra when the Warden is defeated. This ability is used to amass an army that allows him to rise to the top after the world ends.
Very early Forsaken players in World of Warcraft will remember that as early as vanilla, you could run a questline where you gather pumpkins from a farm, gather ingredients for a plague, and give it to captured human. The same disease you were sent to make way back then shows up much, much, much later. As in toward the end of the second expansion later, when Putress double-crosses the Forsaken and Horde by using a very enhanced version of the plague you helped make on the Alliance, Horde, and Scourge indiscriminately at the Wrathgate. In Cataclysm, Sylvanas uses the same plague in a campaign of genocide against the humans.
In the short story "Blood of Our Fathers", Varian, surveying the damage Deathwing has done to Stormwind, finds a small fragment of Deathwing's elementium armor and takes it with him. Toward the end of the story, he uses it as an Improvised Weapon to kill a drakonid.
The weapon you receive when you first start a run in Kingdom of Loathing? It's later used as a meat-smithing component to forge your Legendary Epic Weapon so you can complete the "Defeat Your Nemesis" side-quest.
In Rockman No Constancy, you come across a boss gate that's blocked off early on in Flash Man's stage. Later, in the last stage (a Palette Swap of Flash Man's stage), the same gate leads to the final boss.
The Elder Scrolls actually subverts this rather brutally. In a series where everything can be picked up, from coins to silverware to pillows, there is very often not a use for everything beyond taking up space.
There are also a lot of straight examples though. An in-universe book on Daedra dating back to Daggerfall mentions a 17th prince called Jyggalag, who later turns up as the Big Bad of the Shivering Isles expansion to Oblivion. The Warp in the West from Daggerfall also figures into the mages' storyline in Oblivion as Mannimarco's apotheosis into the God of Worms is what allows necromancers to create black soul gems, while a simultaneously very much mortal Mannimarco is the Big Bad.
The Neverhood has several. Of course, this game is of the Enter Solution Here kind, so things like written codes and such will inevitably come in handy later. However, there are a few more creative examples:
After you leave the very first room in the game, you have no choice but to fall into Willie Trombone's pet flytrap, which promptly spits you out, launching you a few steps across the floor. This behavior is the key to getting into Hoborg's throne room. The game attempts to confuse the player by making this puzzle look like the one involving the first flytrap, where the creature was used to hold a switch ring, but it can't be solved in the same manner due to having two active switches instead of one.
When you first enter the building with the mouse/memory puzzle, the first thing you're likely to do is to step on the floor-pad, which causes the actual mouse on the floor to be sucked away. Nothing interesting happens after that. Seeing the same mouse under a similar device inside Klogg's castle much later on might clue you into what the thing actually does and how it can be used for your benefit.
If you use the lift inside the Hall of Records with the lights on and pull the lever inside the room, you will see an animation of Big Robot Bil getting half his head shot off by a cannon. Guess what you have to do eventually...
In Brain Dead 13, Lance gets the eyeballs, one from each room of one of the twin witches. It later helps Lance out for use as a tetherball weapon against Fritz in the final confrontation.
In Custom Robo, there is a bit dialog at the beginning of the game where the main character receives a watch from his dad. It later is revealed that the watch was part of a memory erasure device which is revealed to be the only way to stop the Big Bad.
In Super Mario Galaxy, there is a red Luma on top of the bedroom dome who tells the player, "I've got a secret! But I'm not telling!" It turns out that his secret is the Red Star power-up, which is incredibly useful for navigating the main hub since it allows you to fly.
In Samantha Swift and the Fountains of Fate the Cortez Emerald of Judgement, picked up right before meeting and escaping from the villain toward the beginning of the game, turns out to be necessary for entering her hideout toward the end of the game.
In The Walking Dead Video Game by Telltale Games, Clementine has a Walkie-Talkie her dead parents gave to her. It's established early that the device is broken and Clementine is only keeping it as a personal memento. However, at the end of episode 3, it's revealed that the Walkie-Talkie was working all this time and that some mysterious man has been communicating secretly with her - promising to reunite Clementine with her parents. The man, himself, would prove to be a Chekhov's Gunman based on a major choice Lee makes regarding an abandoned car full of supplies.
The player spends the first half of Shadowgate picking up a bunch of crap that has no apparent use. Eventually there's a room with a Sphinx who will ask a random riddle. One of the useless items is the answer.
In Phantasy Star you are prompted to go on a somewhat annoying quest to make the Learma Tree grow and collect the nuts it drops. After doing that, the nuts seem to be useless since none of the characters will eat them. It turns out that you use them after finding Lassic's castle, which is on a floating island. They turn Myau from a cat into a large pegasus/cat hybrid that flies the party to the island.
Zig-zagged at one point in Half-Life: During the tram ride in the beginning of the game you can see a four-legged robot walking. A savvy player would immediately think "that robot is going to appear during the game in one way or another". However, it just doesn't... until the expansion pack Opposing Force, where it's stuck in a pool of toxic waste. You have to clear a way for it to proceed, at which point it puts the crate it is holding in a location where you can jump onto it continue.
In Duel Savior Destiny Taiga has the oddly named sword Traitor, which seems odd since it's not even really a true character. However, the meaning becomes obvious in the final route where it turns out Traitor defines himself by being in opposition to God and that's where his name comes from. By the time this shows up, the name probably just seems like random Gratuitous English without any meaning.
Strong Bad's Cool Game for Attractive People has one in the final episode, "8-Bit is Enough." The key used to open the arcade cabinet at the end is the same key used to escape the final dungeon after defeating Ultimate Trogdor.
In Kingdom Hearts 3D Riku's new clothes have the Dream Eater symbol on the back. Eventually it's revealed that he's been acting as Sora's Dream Eater for the entire game.
Sora's new clothes have a prominent 'X' on them. That's the Recusant's Sigil, and it is not a good thing.
Hilariously subverted in Mario & Luigi: Partners in Time. Kylie Koopa discovers a giant key in the Gritzy desert that she believes to be very important. Some time later, you find a giant keyhole on the Shroob Mother Ship and put the key inside, which activates...a trapdoor that drops you out of the ship.
Played straight, however, with Toadbert's Drawing received in Yoob's Belly. The drawing is covered in dirt and considered unimportant apart from recounting what you already knew until later in the Star Shrine when part of the drawing is uncovered to reveal the existence of the second Princess Shroob.
In the same game, Luigi discovers a pile of four Yoshi Cookies intact near the middle of the game. He initially gives one to Baby Luigi when he finds them, and gives one to each of the babies a few screens later to stop them from crying. This means that he still has one left, which he eventually uses in the endgame sequence to pacify Baby Luigi so that the big brothers can say goodbye and resolve the timeline.
Just about everything Inspector Parker finds or picks up in Be Trapped. It will either be relevant to the plot or useful elsewhere, even a seemingly useless envelope from Hargate Prison used as foolscap.
In Baldur's Gate you can get a pair of Golden Pantaloons from a nobleman at the Friendly Arms Inn. They're of no use whatsoever, but if you carry them over to the second game, you'll find a sidequest that, if you follow a certain way, will net you a pair of equally useless Silver Pantaloons, and in Throne of Bhaal you can get a pair of Bronze Pantalets in Sendai's stronghold. If you manage to hold on to all three seemingly useless items until now, you can go to a character in Amkethran and get him to forge for you the Big Metal Unit, the most powerful armor in the series.
In Daryl Gates' Police Quest: Open Season, most of the evidence items acquired throughout the first four days are used to solve various puzzles on the final day.
In Batman: Arkham Origins, you come across the Electrocutioner who claims that the Shock Gloves he has will "kill you, then I'm-a jump start your heart, and kill you again!" Later on, you pick up these gloves from his corpse. They have a very powerful function later on, as they serve as Magical Defibrillators to revive Alfred from cardiac arrest after Bane attacks the Batcave. Near the end of the game, when Batman is faced with a Sadistic Choice to either kill Bane or have him kill both the Joker and Commissioner Gordon by electrical charges in a death trap, the Dark Knight, remembering the Electrocutioner's words, Takes a Third Option by using the Shock Gloves to temporarily stop Bane's heart, then disconnecting the wiring to the electric chair from him and using the same gloves to restart his heart, thus saving all three of them at once. The Electrocutioner is never wrong on these gloves.
Bravely Default: The pendant that Agnes always wears around her neck ends up being very important at the end of the game's story since the main characters need it to have the power to defeat the final boss in both endings of the game.
In Ys IV: Mask of the Sun, the key to raising the Ancient City is the Gold Pedestal you sold to Pim in the first game. Likewise, in the PCE CD version, The Dawn of Ys, the first game's Mask of Eyes becomes an important plot item.
In To the Moon, your characters run over a squirrel near the start of the game. It seems like a pointless bit of Black Comedy, but later on the smell of roadkill turns out to be necessary to trigger Johnny's suppressed memories of the day his twin brother was run over.