A video game trope found mostly in RPGs. Whether it's something you buy in a shop, find in a dungeon, or loot from an enemy's corpse, The Law of Conservation of Detail guarantees that it'll be of some use for someone in your party.
So what does it mean, when you find a giant axe that's too big for any of the characters to wield? Naturally, that the Big Guy is well on his way to join the party. Alternatively, one of the existing characters will get a Plot Relevant Power Up and gain the ability to use such weapons.
This trope usually appears in Eastern-style RPGs, in games where every character has a set type of weapon only they can use. When the game allows choosing player character's class, items usually will be placed in a way that ensures every class will find something useful for them ever so often. In such cases the trope can still occur, when an item is described as being useful for races or classes that are currently unavailable to the player.
Often, this trope is a big clue to the existence of a Hidden Character. See also Interface Spoiler, where existence of a character, item or power-up is suggested by the interface itself.
Note, that this trope only applies to items that are clearly treated as usable by the game's system. A mysterious Plot Coupon that may or may not be crucial for defeating the Big Bad doesn't count.
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Video Game Examples
An interesting one in Tales of Symphonia. There's a point in the game where your party seems to be killed off one by one. During that sequence, you find... a new weapon equippable only by one of the party members that already died. Spoils the moment a bit.
Played more straight in Tales of Graces. A particular Dualize recipe spoils the fact that Richard will rejoin your party, because the Darkshine Crystal can only be dualized with his weapon, and can only be found after he leaves.
Xenoblade has a similar case. A certain party member appears to be killed off early on, but savvy players will notice they had a unique weapon type, which you can still buy...
Double Subverted: Said character uses a completely different weapon type when she re-joins, and can no longer equip the weapons that she wielded before she "died".
In Final Fantasy III, you can find job-specific equipment (books for Scholars, bells for Geomancers, etc.) before the corresponding jobs are unlocked.
In Final Fantasy IV, you can find throwing stars for Edge in Eblan Cave before he joins the party at the end, and buy Paladin equipment before Cecil changes class.
In Final Fantasy V you can purchase spells for Time Mages and Summoners in Walz, shortly before acquiring those job classes.
In Final Fantasy VI, you can find shops that sell ninja scrolls and weapons in areas where Shadow can join the party.
In Final Fantasy VII, you may find a weapon for Vincent way before you can recruit him. And even then he and Yuffie are Hidden Characters, so you'll be encountering better and better weapons for them even if you miss them.
Averted at one point early in the game where you can find Cait Sith's ultimate weapon, but Cloud lets it be because no one in your party has any use for a megaphone at that point and he doesn't even think anyone could use one as a weapon (unlike Yuffie and Vincent's gear).
In Final Fantasy VIII, a Weapons Monthly issue describing Rinoa and Irvine's weapons can be found before either of them is introduced (Rinoa does appear, but you don't learn who she is at that point).
You could also buy ammo for Irvine's gun before you met him. Rather annoying, considering that Squall's weapon was a Gun Blade, which contrary to what one would assume, didn't use any ammo at all.
Final Fantasy IX has claws and forks for Amarant and Quina in the shops way before you even meet them.
Besaid temple's destruction sphere treasure is a staff for Yuna. Tidus hasn't met her at that point, but the inventory screen says her name, albeit in grey text.
Some monsters in Chapter 2 drop weapons that are not usable by anyone. You can carry them over to the next chapter... only to discover that most of them are starting weapons for the new characters.
In Chapter 3 you can buy a weapon usable only by a character that's considered missing-in-action as of second chapter's ending. It sprouted fan speculations about the character returning in a future installment.
Chrono Trigger has a merchant selling you scythes shortly before Magus joins up.
In Baten Kaitos you can find knuckle attacks before Savyna shows up.
In Warhammer 40,000Dawn of War 2 you can find pieces of armour for the Dreadnought way before you get one. Not enough information to guess that Captain Davian Thule dies but once that happens, if you are familiar with the Warhammer 40k lore enough to know what exactly dreadnoughts are, you already know who will be piloting the one in your party.
Subverted in Fire Emblem: Mystery of the Emblem's Book 2: You can get a few axes but never get an axe user; you're supposed to sell them for cash. The remake, which features playable axe users, replaces those bonus axes with simply getting bonus money.
In the first game, you can buy krogan, turian, and quarian armor before you meet Wrex, Garrus, or Tali.
In the second game, you can acquire an upgrade for the "squadmates with geth shield technology" during Tali's loyalty mission, before you meet up with Legion.
In Dragon Age: Origins, as early as Ostagar and even some origins, you can loot large colored crystals that get sorted into weapons and armor inventory sections but cannot be equipped on any your party members. They are actually for Shale, a golem whom you can recruit at the end of DLC-only Honnleath quest line.
Subverted in Betrayal at Krondor, though it's not quite as obvious as some examples. All swords and armor have a "racial mod" of either human, elven, or dwarven, granting a modest effectiveness bonus for the corresponding race. While you do have control of an elf and several humans throughout the game, you never get a single dwarf. The items are still perfectly usable, but the dwarven racial bonus on will never benefit you.
Surprisingly subverted in Diver Down—the very first shop sells an expensive suit of armor that no character will ever be able to equip (and this is a game that does not give Money for Nothing, so you will definitely regret purchasing it.)
In Radiant Historia, you can find claw-type weapons before you get a claw user, and buy weapons for a character who already left the party, spoiling that he would return eventually.
Pretty much the case for every game in the Shining series which generally has multiple character classes and Loads and Loads of Characters. However this can work in it's favour at times because multiple characters can be the same class.
Shining Force III almost always ruins the surprise of a new character as weapon shops suddenly have new types of weapons just before a new character shows up. Most notably with Irene; but as she can die before you recruit her you know there is going to be another character shortly that can use her gloves...and sure enough Kahn appears shortly afterwards.
Subverted however in Shining Force II, where Talos can drop his sword, which can only be used by the Giant class. No Giant ever joins the team.
MOTHER has this in Magicant, where a present containing a Sword can be found. At this point, all the player has in their party is Ninten; the Sword goes on the final member of the party, Teddy, and it just so happens to be his Infinity–1 Sword as well.
Non-Video Game Examples
In chapter 18 of Paper Mario X 2, Link finds a pair of Super Shoes instead of an upgrade for himself, and instantly knows that the upgrade belongs to Sonic.
Referenced in Adventurers! when The Hero panics on discovering a shop that sells a weapon no one in the party uses, deducing that they must have missed a secret character. They meet and recruit the user of said weapons later.