For whatever reason, the player is not allowed to pick up and use the weapons, ammo, and equipment of fallen enemies. Instead, he must either find the same weapons and equipment lying around by themselves, or simply can't pick up weapons at all outside plot events that give them to him. Often this doesn't extend to ammunition; once the player has a gun, enemies with the same gun may well start dropping ammunition for it. This isn't really as unrealistic as it may seem; knowing where your weapon came from means you can be sure it's not booby-trapped or deactivated in some way, but it's fairly safe to assume someone who was just shooting at you is not using a deactivated weapon and is likely to be carrying more ammunition that works in that gun.
This is a staple of the stealth genre, and is often hand waved/justified by the items having fingerprint scanners or some other form of user identification; Truth in Television, in fact; security systems using biometrics or a chip mounted on a ring or bracelet (or even implanted into the owner's hand) have been studied by several gun companies.
In real life, a soldier will be trained to pick up an opponent's weapon only as an absolute last resort. This is because of a laundry list of issues that most fictional depictions skirt around. (A biggy is that when opposing forces are using guns with very different reports, say a US M16 and a Viet Cong AKM, soldiers will tend to shoot at anything that sounds like the enemy...) Additionally, scavenging weapons from battlefields is discouraged because it is fairly common for seemingly-abandoned weapons to be booby-trapped, especially if there is a civilian resistance about. In the case of police and security forces, weapons owned by criminals are evidence and tampering with them could destroy a later court case, as well as becoming an easy cop-out point for lawyers. On the other hand, in that absolute last resort when you're out of ammo for your own weapons and surrounded by enemies, and if escape or surrender are not options, then no reasonable person would criticize you for using a captured weapon. Similarly, in the game context, it will feel unreasonable if the game prevents you from picking up the gun from the dead Mook next to you.
Within gaming tropes, contrast Exclusive Enemy Equipment and Randomly Drops. Related to Good Guns, Bad Guns. Compare Statistically Speaking.
In other media, contrast In Working Order.
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While the player character in Overlord is unable to pick up the enemy equipment, his minions can. And they'll pick up a lot, causing some laughs when you realize some of your minions are wearing pumpkins, beards (yes, beards), and a flower in a patch of dirt as headwear. Could be considered justified as you ARE the Evil Overlord, and no self-respecting one would be using such non-Doomy Dooms of Doom objects for their personal armament, plus the minions will happily fork over gold and other things that would be valuable and useful to you.
This trope is avoided somewhat in The Legend of Zelda: The Wind Waker. Some enemies do in fact drop weapons that you can pick up and use, ranging from simple clubs to a BFS that's more than twice the length of Link's body.
But effectively played straight with Phantom Ganon's sword, which is only dropped in rooms where there are no other enemies anyway. Which is secondary to the fact that you don't want to pick it up anyway, since watching how it falls is a puzzle hint.
Twilight Princess averts this only when it comes to arrows. Much like the Oblivion example, you can pick up enemy arrows that get stuck in the ground before they fade or burn away, and recover your own fired arrows from an enemy if you can see them sticking out of it.
And you get the Gale Boomerang and Ball and Chain from beating the two minibosses that use them.
Skyward Sword plays with this trope a bit. In the fourth dungeon, Ancient Cistern, you obtain a whip which allows you to retrieve items from afar. Unfortunately, it can't temporarily snag weapons from enemies, instead only stealing Monster Horns (for upgrading your equipment) from certain Bokoblins. Later on, the boss of Ancient Cistern (Koloktos) must be defeated by disabling its limbs, which allows you to pick up one of its swords (which are able to smash through pillars) and go buck wild on it. Unfortunately, you can't take the sword with you outside of the boss room.
This game plays it straight in the case of the bow. Enemies use them from very early on in the game, but it is the last item that Link acquires.
Played straight in all three of the original Tomb Raider games by Eidos for Playstation. In TombRaider, you can't take any of the weapons used against you by your human enemies (Pierre's pistol, The Cowboy's magnums, The Kid's uzis, Kold's shotgun; they all instead drop your version of the same gun when killed). This trope is epitomized in Tomb Raider II, when you fight and kill entire armies of human enemies, and the weapons they use includes silenced pistols, shotguns, automatic rifles, harpoon guns, and oversized uzis, which all stay clutched in their cold dead hands when killed, inaccessible to the player. Tomb Raider 3 continues the tradition, notably in the level Area 51, where upon jailbreak and getting Lara's signature twin pistols, she can kill a sniper guard wielding a laser sighted automatic rifle and can't take it for her own (Lara has the similar MP 5 weapon which she must pick up separately).
While games from Crystal Dynamics notably avert this trope, Underworld plays it awfully straigh. Lara's arsenal is limited to her own weapons. She can't get any ammunition for them (as a more realistic approach than finding mags for modern weapons in ancient, long abandoned sites) nor equip weapons dropped by her human enemies. The latter one is quite annoying, as you burn through her ammo reserve pretty fast while fighting humans, but can't get any of it back.
The Quest for Glory series was notorious for this trope. Even if you were equipped with only a dagger and leather armor, and you just killed dozens of enemies carrying scimitars, spears, maces, scale mail, shields, ball and chains, etc. they would invariably be too 'damaged' or 'worthless' for you to pick up, if the game even acknowledged their existence in the first place.
Beat 'em Up
In Undercover Cops, the playable characters cannot wield knives, bottles, bats, or axes. This is kinda justified considering they can all shoot energy beams and wield weapons 2 or 3 times their size.
First Person Shooter
Many, many First-Person Shooter games. Nowadays it's less prevalent: the player character can pick up and use enemy weapons. In less modern shooters, however, defeated enemy grunts would often lie as corpses on the ground with their weapon in plain view, but you'd be unable to take it unless they spawned the appropriate weapon/ammo item while dying. As noted above, this isn't necessarily unrealistic.
Averted in the Granddaddy of them all, Wolfenstein 3D. Every human enemy, save for the bosses, dropped their weapon when killed, giving you that weapon if you didn't have it before or adding to your ammo count if you did. The game even starts with the scenario that you take the pistol from a guard you shanked.
Doom likewise averted this trope for the most part in that enemies that actually use weapons (that aren't granted to their bodies) drop them, though the standard rifle-using zombies only drop ammo. For your pistol. The apparent reason for this is the player's starting weapon was originally a rifle early in development, but was changed to a pistol without changing the sprites of the enemies wielding the same weapon.
The Brutal Doom mod subverts this somewhat. Killing some enemies (like mancubi and revenants) a certain way allows the player to steal and use their weapons (though they tend to be Awesome, but Impractical.)
Golden Eye 1997 for the N64 averts this trope completely until it comes to dual-wielding. Even if you have one pistol, if you kill a guard also using that pistol, you only get ammo for picking it up - you must find a guard using two such pistols, kill him, and pick up both of the dropped weapons in order to dual-wield them.
Most Tactical Shooters will forbid you from carrying enemy weapons except to take as evidence. The reasons for this are simple: they are either poorly maintained or are different from what your team is carrying, which in Real Life will lead to friendly fire incidents.
It's zig-zagged a lot in BioShock. The weapons of Leadhead Splicers are blown away from the corpses when killed, letting you obtain them or refill your own's ammo. Nitro Splicers and Rosie's weapons are too cumbersome for the character to carry, even though they can be partially cannibalized for materials later on. Adam and plasmids cannot be gotten from any enemy, since only Little Sisters are capable of extracting them.You can't get weapons from robots, turrets and cameras, but you can get bullets, rockets and Film. Melee weapons are either too heavy or more crude (and thus weaker) than the wrench you start with. Finally, near the end of the game, you are required to find a Big Daddy's suit: an audio log you find in the facility explains that the suits of the Big Daddies you find around are molded to them and thus cannot be reused, forcing you to look for a new one.
In Call of Duty: World at War, the player will see many Japanese officers with a Katana, sometimes even using them, but they can not be pickup up or used by the player after killing the officer. Otherwise averted throughout the series: if someone can be killed, their weapons can usually be looted.
Pathways Into Darkness was unusual in that YOUR equipment was the unusable stuff at the start of the game: due to making a hard landing thanks to a defective parachute, your M16 gets a bent barrel, the bag with all the spare ammunition is lost in the jungle somewhere, and your Colt .45 sidearm is empty for some reason. Oddly enough, when you reunite with your squad or rather, the remains of your squad all of their M16 rifles have bent barrels, too!
While the first Halo game ignored this trope for the most part, some items (FRG, Sword, Shield, Wraith) still cannot be used. The second and third games clean this up for the most part.
They also justified why you couldn't use those weapons. The FRG and sword had self destruct devices in them and blew to pieces as soon as dropped. There was no way to snipe the elites out of the sealed-shut wraith, and hijacking hadn't been programmed yet. The guns the hunters use are grafted into their armor and therefore impossible to take. The jackals' shields however, were never given any justification, and Halo 3 came and went with still no ability to wield them.
It was possible at one point to snipe an elite before he got into his wraith. You still couldn't drive it though. Unless you hacked, also revealing it to have the same crosshair as the rocket launcher.
In-story, the Elites will rather fight bare handed rather than used a loaded human weapon right next to them. This doesn't stop you from picking up human weapons when you play as the Arbiter in Halo 2 though.
The Shadows in Halo 2 and the AA Wraiths in Halo 3 are also unusable by the player.
In Half-Life 2, after you're weapon-stripped, your only remaining weapon gains a weapon-destroying effect itself... which means every weapon dropped by dying enemies is disintegrated before you can grab it to rebuild your arsenal. Not that you really need to.
Half-Life has an interesting example with the Hive Hand weapon. Unlike every other weapon the player can't retrieve it from the corpse of an enemy that wields it, because it's literally attached to them. There are two Hive Hands that have been previously removed and the player can acquire, but other than that they can't be used.
Gordon has yet to have been able to use a Combine sniper rifle.
Particularly noticeable in the Jedi Knight games. Despite the fact that the player is allowed to pick up any gun dropped by dead enemies, they are for some reason unable to pick up dropped lightsabers.
Much like the sissy defaults for dismemberment, this can be overcome by the flipping of a few flags in one's INI file.
In Resistance: Fall of Man you can't get the fireball shooters used by Chimeran Titans when they die. Justified twice over, as said guns are as big as you are... and Titans die when their cooling units overload and explode, blowing them apart. It similarly justifies not being able to get the weapon Slipskulls use from their corpses by having it mounted onto their arm with metal bands. There's no obvious reason the Arc Cannon can't be recovered from Hardfang corpses, though — it's just not there when you try. If you look closely, they literallyvanish in a puff of smoke; no, there's no apparent reason why.
In the game's New Game+ mode, both the Slipskull weapons and the Arc Cannon are available to the player, alongside a couple of fancy new pieces of kit that you had no way of knowing existed. Of course, it might have been helpful to actually tell the player this at some point...
Ghost Recon lets you choose a set of weapons at mission start, but you can't use the weapons your enemies drop after you kill them. In Advanced Warfighter, you can at least scavenge the ammo out of them if they're the same caliber as one of your weapons, while Future Soldier lets you grab dropped enemy weapons as much as you want.
In Team Fortress 2, weapons dropped by enemies act like medium-size ammo boxes, giving you 50% ammo, 100 metal (Engineers), and 50% cloak (Spies). The exception to this is the Heavy's Sandvich, which restores health to the person that picks it up, the Scout's baseball, which can be used by other Scouts, and the Engineer's toolbox, which fills ammo completely.
However, you cannot actually obtain the items collected this way, as in have them in your inventory. While the weapon drop system renders this a non-issue, it would break the fandom apart if you could get a hat by simply collecting it off of a corpse. The game does make one exception though: dominating someone currently wearing a Ghastly Gibus or any of its variants would grant you a Gibus of your own through the "Ghastly Gibus Grab" achievement.
Generally averted in Borderlands. Not every enemy drops their gun, but it IS a common occurrence. As each gun has many randomly generated elements, and some rare guns have distinct effects, you can get a hint or even know outright what weapon the enemy will drop when you kill them. Characters cannot equip armor (Besides a personal energy shield, which DO drop from every shielded enemy) however, and therefore you can never take the armor worn by enemies such as the Crimson Lance.
Averted to a degree in Deus Ex. The game seems to be inconsistent at times about whether the guy you just stunned/kill will drop ammunition or grenades, for example, but almost everyone and their dog seems to have spare combat knives on hand, whether or not they pulled one on you. Very rarely do you get to actually scavenge a proper firearm, at least early on.
Treated rather strangely, actually — the game simply will not allow you to have duplicates of non-stacking weapons. If an enemy carrying a pistol doesn't have it when you search the body, drop your pistol and search him again — he will often suddenly have a pistol. If you pick yours up again, you will still have only the one you took from the enemy (yours will vanish, which could suck if you've been upgrading it...)
Played straight in Rainbow Six (or at least the earlier games), although justified. In addition to practical reasons listed above, most missions require suppressed weapons, which the bad guys rarely have. No ammo drops either, as enemies rarely use the same ammo, and when they do, it's generally a non-compatible magazine style. Annoyingly, (unless I'm mistaken) you can't get ammo off from your fallen comrades either, even if they are using the same weapons. Made slightly more annoying since there are no One Bullet Clips.
The World War II Medal of Honor games, such as Frontline, prevent the player from picking up weapons from enemies. This is usually for gameplay purposes: the player would either have no need for the weapon (like a K98 bolt-action rifle) because superior ones are available in large numbers, or the enemies all carry the same guns as the player and simply provide ammo. However, almost all enemies will still drop ammunition for American weapons, suggestion that either the M1 Garand is able to chamber both .30-06 and 7.92mm Mauser, or that the K98s are all loaded with .30-06 rounds.
In Syndicate (2012), you can't pick up shields dropped by enemies or the Subverters' Molotov Cocktails. Other weapons are fair game though.
Played painfully straight in all Delta Force games. Your choice of weaponry throughout a mission is the guns you picked or the guns you picked. Friendlies' guns or even their ammo can't be collected either.
Averted in the first PlanetSide, which let players loot other player's corpses to steal their weapons (which could then be stored in personal storage lockers accessible from any base) regardless of empire, allowing a Terran Republic player to pick up a New ConglomerateJackhammer and start blasting, for example. Players could also hack and steal enemy vehicles if they had the certification for it. Played straight in the sequel, where players cannot loot enemy (or ally) loadouts for their weapons, or even ammo.
Megaman Zero 4 averted this. Zero's new weapon, the Z-Knuckle, is some kind of energized hand attachment which enables him to literally tear weapons off of enemies and use them himself. There's a huge variety of weapons and gadgets he can steal this way, but he can only use one at a time, and most of the projectile weapons have limited ammo (which a certain upgrade part can regenerate).
In Pitfall The Lost Expedition, Harry passes by crates full of TNT throughout the game and is assaulted by enemies that throw it. You cannot use it yourself until a friendly character hands you some during a cutscene late in the game.
Justified in Iji, where part of the reason many later enemies explode is to stop enemies taking their weapons. You can also partially avoid this by hacking them so they still leave some/more ammo though.
Avoided entirely in NetHack: if an enemy is using an item, you can loot it off their corpse when they die. But they don't all leave corpses behind—which is far worse, since you'll need food a lot more than you'll need (say) even more rusty pig-iron broadswords.
Ancient Domains of Mystery has moloch armor, occasionally dropped by one of the nastier enemies in the game. Technically wearable, but heavy enough to crush many characters to death, and it slows you to a crawl. Though the PV bonus it gives is admittedly nice.
Role Playing Game
In Gothic players can always take and use the weapons of defeated enemies (even huge orc-axes), but never their armor. This leads to the best armor from the Old Camp being inaccessible. There are plot reasons why you'd never normally get it, but still...
It is interesting to note that you can't get the armor, but you could definitely wear it, if you got it through other means (by using cheats, for example). Every armor on every human enemy is an actual, wearable item with its own stats — you just can't take it from them. This applies even to things that look like they should be part of the NPC model instead, like Xardas' black robe.
This was changed in the sequel, where this and some other kinds of "armor" don't actually exist as separate objects anymore.
This is also present in so many video game RPGs that making a list of them would be useless. You can have a random encounter with an enemy who is a knight with sword and shield, wearing armor, but you're never going to get the sword, shield, or armor unless it comes as a random drop. Exceptions include:
The Elder Scrolls games: Since Morrowind, you can access an enemy's inventory and take all of their equipment, including their weapons, armor and ammunition. You can even loot some of the arrows you shot at them! Some enemies, however, show equipment on their models that is not actually in-game equipment and therefore cannot be looted.
Every single piece of enemy equipment in Titan Quest is a usable item. If they have a shiny weapon, you will get it. However, most pieces are far below normal quality.
Vagrant Story shoves this in your face, painfully. You can actually see each individual piece of equipment that each enemy has equipped, but you have only a tiny chance of any piece of that equipment being a Random Drop and hence obtainable. The game doesn't attempt to explain this.
Betrayal at Krondor allowed you to loot everything off of your fallen enemies, from weapons and armor to their rations.
An earlier counterexample is the Ultima series, in particular Ultima VI and 7 and their related sub-games, in which every single lowly guard drops his sword, armor, et cetera when killed. The effect is that you quickly stop looting the junk because you simply can't carry two dozen sets of armor around.
The Baldur's Gate and Icewind Dale games are also counterexamples since humanoid enemies will drop their equipment on death (except things for which it would not make sense, like ghosts. The final bosses are also exempt). The games even keep track of the amount of arrows in opponents' quivers and the like: The faster you kill an archer the more arrows you can get to use yourself, and enemies can run out of ammunition and be forced to engage you in melee. However, it's also hardly worth the effort to pick every sword and armor up since they weigh a lot and don't sell for much. Normal ammunition can't even be sold at all.
In Fallout 1. all armor save for the Powered Armor worn by the Brotherhood of Steel could be looted. In Fallout 2, armor wasn't lootable; apart from the game-balance issues, presumably the idea of shooting the enemies through their armor and then using it yourself feels a tad unrealistic. The Powered Armor exception was pointed out in-game too: One of the paladins of the Brotherhood of Steel mentions that you'll never encounter a non-brotherhood soldier in their trademark power armor since with it fully encasing the wearer, the latter dying means the armor has been shot to unwearable bits.
Frank Horrigan in particular has a special unique gatling laser and knife that are the most powerful of their weapon types, but can't be pilfered off his corpse. Justified in that he is the Final Boss, but then again the player can continue with the game in the Playable Epilogue anyway.
Certain robots also have built-in weapons, like Sentry Bots and their rocket launchers, and turrets and their dual miniguns. Not being able to take the weapons off of them is understandable, but not being able to pry them open after they're destroyed and take their remaining ammo is less so.
In Fallout 3, anything that the enemy carries, wears, or uses, can be stolen. Early in the game, it's a good idea to leave your dead enemies rotting in their underpants. This extends all the way up to the Enclave and Brotherhood armors, which you can easily pilfer off the dead. However, no armor you find this way will ever be in 100% top condition. You can even loot the power armor, but you can't use it until 2/3 of the way through the main story quest. Unless you have the Operation Anchorage DLC, which will not only give you Power Armor training much earlier, but also a nigh-indestructible T-51b. Although played straight during the VR simulation, where enemy weapons disappear along with their corpses.
Which is justified, as it's an old military simulation, and for the reasons listed in the intro their soldiers would have been trained not to pick up and use enemy weapons anyway, so why should the simulation let them do so?
Continuing the tradition is Fallout: New Vegas, where the Scavenger World motif remains in full swing. Notable for actually applying faction reputation onto any armor you might pick up off enemies, so early on, where the main human enemies are two-bit thugs with dynamite who also happen to be universally detested by all other major factions in the game, it now becomes a consideration as to whether one should go about using what you loot or pickpocket off most humanoid enemies in the game. The Old World Blues DLC takes this to a rather weird inversion where NPCS will carry weapons the player can use, but that they can't. For instance, a robot without the hands or arms to use a melee weapon will somehow have axes in its inventory, and a robot dog can be found with a powerful electrolaser rifle, again, without the necessary appendages to use it.
Both Fallout 3 and Fallout: New Vegas play this straight with certain unique clothing and weapons used by NPCs, which cannot be looted or stolen and can only be acquired using console commands in the PC version. Also subverted in that some unique NPC armor and weapons can be looted but the version that the player character picks up is slightly weaker. In other cases, only the standard version of a weapon is available in an NPC's inventory when being pickpocketed or after they are killed, as opposed to the unique variant that they normally use.
This is present to some extent in Knights of the Old Republic, where most weapons and armor (especially blasters) cannot be collected, but ancillary items (medpacks, stims, grenades, etc.) are commonplace drops. Since loot is randomized, this makes sense, but it also leads to the odd situation of an enemy dropping an item he isn't even carrying, such as Dark Jedi dropping a blaster rifle. Boss battles are a major exception to this trend, but this is the case sometimes even then (e.g. the Sith governor of Taris wields a double-vibroblade that can't be scrounged).
World of Warcraft has a notable quest where you need to collect weapons from specific monsters (which you could actually use, if you so wish), but as with all quest drops, the chance to get one is far lower than you'd expect. In fact, the chance of another weapon is higher.
And humans that don't have skulls. You'd think you could recognize which do by them not having floppy organ-sacs on the top end of their necks...
The standard Hand Wave is that you, in the process of killing the mob, destroyed the body part in question. In classic Blizzard style, this response fits all scenarios, ranging from the plausible to the... not.
Hard to imagine destroying a troll's ears or tusks in the course of killing it.
Flash RPG MARDEK is released intermittently in chapter format, with items and stats impressively being carried over from chapter to chapter. Unfortunately,chapter 2 stacks you with staffs - Unusable Friendly Equipment because no one in the act can actually use staffs, no matter how good they are.
However, the actual trope is played straight, as a number of enemy characters wield weapons none of your characters can, including knives, walking sticks, and guns.
Wizardry games shows names of opponents' weapons, but those are just strings, not really equipped items and as such may or may not be reflected in loot. In Wizardry 7, T'Rangs poke Shock Rod, Stun Rod and Psi Rod into PC. Shock Rod has Drain (stamina) 20% in their hands and Drain 50% in PC's. Cool, but as a weapon it's mediocre. Stun Rod is Paralyze 65% Drain 35% for them, but only Paralyze 20% Drain 75% for PC (same damage as for weakest foe armed). Attacks with "Psi Rod" are even more dangerous, but... oops, no such equippable item in game. The same in Wizardry 8, with some numbers changed.
EVE Online mostly averts this. In the case of player ships, a subset of the gear that the killed player was using will drop, and can be used by anybody with sufficient skills and a capable ship. However, NPC drops are only loosely related to the equipment they may have been observed to use during the fight.
Played completely straight by Rogue Drones, which only ever drop crafting materials, and The Sleepers who never drop their overpowered armor plating, missiles, or beam cannons.
Averted very, very well in Siege of Avalon. You can strip dead enemies down to their underwear (and sometimes take that too, though this doesn't change the dead enemy model having underwear) if you feel like it, though actually carrying that equipment in your bag can be problematic due to a bag of limited size and only being able to wear so much stuff at once. The same items can be thrown on the ground (and stay there until you come back for them!) if you decide you don't actually want them, but they can't be put back on the corpses. Unfortunately. That could have been funny, dressing up a dead enemy whose people are on a religious rampage against everything your people have touched in your old clothes and a silly hat...
The engine of Neverwinter Nights allows for this as well; any equipment a humanoid enemy possessed can be flagged as droppable, and looting the armor or weapons of a dead enemy will actually remove it from the corpse's model. The official campaigns go back and forth on this; generally, the expansions are more likely to let you loot enemy equipment, but it varies from encounter to encounter. Unofficial modules, of course, can run with it any way the creator likes.
In City of Heroes, you can't pick up any enemy weapons, even when your character is using guns rather than shooting lightning out of their eyeballs. Which is a shame, since a lot of those guns look extremely cool. Fortunately, there are various unlockable skins avaliable for those with the Assault Rifle powerset, and for the rest of us one can sometimes get a chance to wield a gun through temporary powers with limited ammo.
This trope is especially noticeable when you consider how lacking in offense some Archetypes are, particularly in the early game. A low-damage type like an Earth/Empathy Controller, for instance, could certainly find use for that fire axe or sword no matter where they are in the game.
In designs made with Unlimited Adventures, by default, goblins and brigands and other humanoid enemies have a lot of items that they drop after death and that can be collected. However, this just means that after making short work of a gang of orcs, you are faced with a giant pile of useless items (some literally useless, like the basic helmet, which does not do anything at all) which, if picked up, will just encumber your PCs until you find a shop and sell them for tiny amounts of money (unless the design author turned up the prices to Game Breaker levels.)
In the PC version, you can mod the ini file to make enemy drop their weapon (noticeably the Heavy Machinegun from enemy Guard Mech), but the weapon will disappear after a while, in your hands, even if you are in the middle of using it to blast at enemies. It's also an incomplete and incredibly buggy feature, so enable it at your own peril. The YMIR gun is Awesome, but Impractical, and most of the other guns aren't worth picking up.
In Dungeons & Dragons, many magic items are usable only by characters of a particular alignment (Good, Evil etc.), and generally players are opposed in alignment to their enemies. This prevents the use of some NPC item by player characters.
In 2nd Edition D&D the magic weapons of the Drow (underground evil elves) turn to dust after being exposed to sunlight. Drow equipment based on radiation magic works just like magically enhanced items, but neither needs to be actually enspelled nor can be disenchanted as common variety. This disintegration doesn't bother Drow themselves, as they raid surface rarely and only at night anyway.
In 3.5ed, PCs, especially Rogues and Bards can train the "Use Magic Device" skill and somehow use it to fake an alignment... or race... or class. Making such gear merely difficult to use.
On the other hand magical equipment in D&D generally tends to grow or shrink to fit the wearer "from halfling to ogre size", effectively eliminating the most realistic reason for a player being unable to use enemy equipment.
And now, in 4th Edition, holy symbols — which used to be trinkets that did nothing but allow you to cast many of your divine spells — are now as scalably powerful as any magic sword, suit of armor, etc. However, this mechanic highlights the edition's Gameplay and Story Segregation: it's not really a moral qualm for any good fighter or even paladin to wield a sword or wear armor with Spikes of Villainy, but now clerics are left the option of either upgrading to a defeated enemy's evil-deity-specific symbol to get a power boost (even though it wouldn't actually change their religion), keeping their old junk, or going through an hour-long ritual to convert the evil symbol into a sort of "raw magic" that will only go 1/5 of the way towards creating the good version of the same item.
Although this seems to be at best a matter of personal taste more than an actual instance of this trope — 4E holy symbols are by all appearances functionally generic divine-caster implements in terms of game mechanics, and if you can use one by virtue of being the right class, you can use them all (though only one at a time, of course). Or the party can just happened to know about a simple ritual that swaps holy symbol properties.
In the Tabletop RPG Deadlands: Hell On Earth, the Black Hats use vehicles and weaponry equipped with self-destruct devices that trigger if anyone without an identity chip tries to use them. Said chips are surgically implanted in the Black Hats, and naturally self-destruct if anyone tries to remove them.
In Resonance of Fate, you can't pick up any of the guns from dead human enemies. You'd probably want to — their handguns do about 100 times more damage than yours and they have shotguns and assault rifles which you can't get at all — but they all vanish with the enemies when they die. Some large enemies will drop weapons, but they're broken and not human-usable anyway so they can only be used for Item Crafting (how the Tinkerer manages to make tank-sized weapons into normal gun parts, in such a way that you can disassemble the gun parts and get the tank weapons back is left unexplained).
Mostly averted in Final Fantasy XI, as many beastmen that you fight use weapons and shields that actually have the same texture models as player-usable equipment. On the other hand, some beastmen dropped items, such as the Quadav Helm, explicitly say that playable characters cannot wear them, and are generally either used for Twenty Bear Asses quests or for synthesis materials.
Some Monster Rancher games have unusuable enemy monsters. In 2, there was a series of wild monsters whom you could fight and obtain cards for, but never own. In 4, in addition to your rivals having monsters you can't, several of the game's bosses are actually old monster species from past games—with proper movesets, even, although you're still not allowed to use them. In Monster Rancher EVO, this gets downright silly, as some of the enemy monsters are perfectly normal things you could theoretically get, but aren't allowed to. For example, a Piroro/Gitan crossbreed—it's an opposing monster, and Piroro and Gitan are in the game, but you're not allowed to fuse them.
Browser-based nation simulation game Cybernations has a variant: when troops engage in ground battles, the winner loots the loser's equipment, which is instantly converted into money.
Averted with nearly all weapons in Dwarf Fortress, and the ones your dwarves can't use themselves can be used up to ten at a time in traps. Armour used to play it straighter, as dwarves couldn't wear any other race's armor, but now they can use goblin or elf armor (because all three are the same average size). Kobold and human armor is still unusable.
One curious example is in the weapons that are too big for dwarves to use. With the update that enabled individuals to vary in physical characteristics, some dwarves can get big enough that they SHOULD be able to use those weapons. But they can't, because the weapon only considers the average size of the creature's species. This is a bug, as individual sizes DO factor into the check for whether a creature is big enough to wield a weapon one-handed.
The explanation given in Metal Gear Solid 3: Snake Eater, which is set in 1964, is that Naked Snake doesn't trust the reliability of weapons that may have been poorly maintained, instead preferring fresh weapons from armories.
In Metal Gear Solid 4: Guns of the Patriots, enemy guns are once again ID-locked. Fortunately, one of the NPCs this time around is a gun launderer who can unlock them, for a price; you also can still pick up their guns anyway, to both get ammo from it and the currency used to unlock guns from said gun launderer.
At one point during MGS2, the protagonist must infiltrate the enemy's base by disguising himself as a guard. But he can't just kill one and take his uniform, he must be given one during a plot event - at the very least, this is explained in that the guards in the core area Raiden needs to infiltrate are issued entirely different gear from the guards he's encountered in the surrounding shells.
The same thing needs to be done in MGS3, but you must steal the uniform from a specific officer because you need his security clearance and it just so happens that you have a mask that matches his face. If you wear the mask and talk to Sigint at the beginning of the game, it's heavily hinted at that the mask was made to impersonate the exact same officer.
The Chronicles of Riddick: Escape from Butcher Bay also does the DNA-gun thing. Enemy guards actually do drop their assault rifles when killed, but the rifle electrocutes you if you try to pick it up. Guards with pistols and shotguns can be freely liberated of their arms once downed, though.
In Assault on Dark Athena, the rifles are no longer DNA-encoded, which means when you find a merc you can take his weapon no matter what. However, the most commonly encountered enemy, the Ghost Drones, have their rifles surgically attached to their arms. Semi-averted in that you can use their guns while using their body as a meatshield, but this inhibits your ability to move.
In the Splinter Cell series, Sam Fisher, for whatever reason, can't use the guns of fallen enemies. For ammo and weapons, he must find them laying around by themselves. This becomes ridiculous in the Xbox version of Double Agent, where at one point a choice made earlier in the game can net Sam a pistol carried by a guard... but it is impossible to take the pistols from any other guards!
This was lampshaded at one point in Chaos Theory, where Sam can find an email yelling at a mook for ordering the wrong ammunition, that only Sam can use. By extension somewhat justifying the trope, given that all the weapons shown are modeled on real weapons, very few of the enemies would carry ammunition for the weapons Sam uses. Also, no one in their right mind would trade a FN F2000 with suppressor and grenade launcher (specially designed for firing less-lethal rounds) for a terrorist's AK-47... except for Sam in Conviction, where he can now do just that whenever he wants.
It should be noted that Splinter Cell is a stealth game, and most of the guards' guns have no silencer. Using a loud machine gun that would attract every guard in the complex to your position wouldn't be very productive.
In a rather useful aversion, Oni not only allows you to strip and disarm enemy weapons, but the lack of One Bullet Clips means that it's advantageous to do so just after an enemy reloads. Likewise, if you see an enemy using a forcefield (the kind that stops bullets, but not punches), you can pick it up after you drop them, with same level of power remaining. Hence, making a punch to the face more effective, as it makes their equipment much more usable.
Nicely averted in Syphon Filter. Normally, shooting someone in armour will destroy said armour. However, if you shoot them in the head, the armour is intact and can be looted.
Warhammer 40,000: Chaos Gate does not let you pick up Chaos Weapons on the grounds that (to quote the manual) "No self-respecting Space Marine would deign to touch a weapon used by a minion of Chaos". In fairness, canon would insist that the weapon would turn against the righteous bearer or cause him to sprout tentacles or something anyway. Chaos-tainted artifacts are the ultimate in non-user-safe.
Which is odd, because Logan Grimnar, head of the Space Wolves is explicitly stated to use a Daemonic axe he looted from the corpse of a chaos champion, and mastered using only his willpower. Guess the rules don't apply to Chapter Masters. Then again, the Space Wolves may as well have their motto be "Screw The Rules." Logan was looked upon as out of his damn mind even by his fellow Space Wolves when he decided to do it, before they realized "holy shit it WORKED."
Said axe was melted-down, reforged and reconsecrated.
Warhammer 40,000: Fire Warrior averts it as all weapons can be used indiscriminately. La'Kais doesn't suffer any of the consequences for using Chaos Weapons since the Tau have an innate resistance against the Warp.
And then there's the Ork weapons, which only work for them because they actually shouldn't work for anyone at all. That's the joke at least. The literature's consensus is that, while crude, ork equipment is usable by non-orks, but the orks' psychic field makes it work better.
The Tau also got something for their Battlesuits and Stealthsuits as they have a fail safe system that will fry any human/ork who tries to steal the armor. One guardsman learned it the hard way.
And, at least in human case, it would be pointless anyway, as somebody openly using reverse-engineered alien equipment would piss off either Adeptus Mechanicus or Inquisition, or if less lucky both of 'em.
And for everything else, it clearly stated attempts in reverse engineering let alone using it has ended in the grisly demise of the user. Case in point are Necron Gauss Weapons which vaporized techpriests dabbling with it.
Dawn of War 2's first expansion features an interesting example: if Techmarine Matrellus is the traitor, he drops his corrupted Servo-Arm on defeat. None of the other playable characters are Techmarines, so none of them can use it. It's only good for cashing in, and doing so gives you back a weapon you can use.
Jagged Alliance 2 features an aspect of this trope. Late in the game one will get access to an experimental rifle that fires small explosive rockets. This features a trigger mechanism that reads the fingerprint of the first person to use it and then limits its use to that person. Said mechanism can be reset by a PC or NPC with sufficient skill in electronics.
The beginning of the game was also made artificially harder by the fact that, while your mercs started out with peashooters, the enemy Red Shirts had rifles and body armor that would much more often than not disappear with them. It's only after you finally did get access to your own supply of rifles that the Red Shirts would start dropping them regularly. The v1.13 mod includes a game mode setting that allows every item the enemy mooks are carrying to be picked up...
One of the ways to get around this was to literally STEAL the equipment from a desired enemy by using one of your more stealthy mercs to sneak up on the enemy, loot him, and run.
Shining Force 2 has a few bosses that can drop items. These items can't be equipped, but one of them can be used to cast spells.
In the X-COM series, all equipment used by the aliens will show up as an 'Alien artifact' and will be unusable. It is still possible to interact with these items... usually by accidentally blowing them up, which prevents you from looting them post-battle. After researching the specific weapons/items, you will then be allowed to outfit your squad with those weapons in addition to looting them off the corpses of your enemies. This is basically how you "level up" your weapons as you precede through the game.
To lesser extend, alien ships aplies as well. You can secure several intact flying saucers trough the game. You can can pillage them for artifacts and raw materials, but you are never allowed to keep one, even if it is superior to your fighters/transports and you have scientific expertise to make it fly.
In XCOM: Enemy Unknown, alien equipment is unusable due to strange ergonomics. Plasma pistols are designed to be mounted on the wrist of something with really narrow arms, heavy plasmas are too cumbersome to use without a few new grips, and so on. As soon as you get the gear back to base, your engineering team adapts the gear for human use (adding grips, stocks, proper sights et.c.) free of charge and off-screen.
Also, the weapons of killed enemies will self-destruct, even if the death didn't involve getting shot at or blown up (e.g. Mindfray), implying a failsafe mechanism meant to prevent the weapons from being captured. Considering every alien has a number of implants in their bodies, the weapons could be linked with the implants to that effect. The only way to capture intact weapons is to stun the alien.
Ditto for The Bureau: XCOM Declassified (which is a Third-Person Shooter), where Outsider weapons are destroyed when their wielders are killed. This is even mentioned by the Bureau's resident Mad Scientist when he gives you a bracelet designed to allow you to map your biometrics to alien weapons. He tells you that you must find un-mapped weapons, though.
There are often weapons which enemies in Fire Emblem are seen using, but the player cannot actually obtain.
If you cheat to get them, they will usually still work, the most notable one is the final bosses magic book in FE7 which would let anyone use magic. Nils could actually do damage!
This trope is justified with Fire Emblem's beast enemies. In these cases, the "weapons" are fangs, claws, or other parts of the beasts' anatomy.
Although in Fire Emblem: The Sacred Stones, you could take monster weapons with some glitch abuse. It was pretty much the only way to teach anyone but Knoll or Ewan dark magic, and the only way to let Myrrh attack at all once her stone broke.
A common staple in Nippon Ichi games, though considering everyone and their grandmother (sometimes coming up as a storypoint even) is portrayed as having Cthulhu-sacking prowess, it's generally accepted they simply annihilated the other person's equipment along with the entirety of the enemy. Which doesn't explain how it comes back when you get them revived at the hospital, but there you go. If you see something you like, you must either steal it with a special item (it doesn't have to be used by a thief, but it's much harder otherwise) or capture the enemy and take the items away.
The Disgaea games in particular have Geo Panels that can clone your characters - the clones are hostile and replicate the original exactly, down to the equipment. It is impossible to steal their equipment even with the specialty items listed above, and only a weapon in the third iteration has the potential to knock only one of those items off the enemy when they do die to it; if you could freely steal equipment from the enemy, it would (much sooner than usual) snap the game in half.
4X game series Space Empires, and possibly other games that use tech trees, allows you to capture enemy ships and study them for new tech. However if they have Ancient Ruins, or Racial, technology you can't use it because only empires with that tech tree have access to it. If you're lucky you might have found those ruins as well or are the same sort of race.
Sword of the Stars got Boarding Pods and the ability to capture enemy ships in an Expansion Pack. But the captured ships will inexplicably vanish after the battle is over so you can't use their (possibly superior) technology.
Sniper Elite V2 is weird about this — you have access to everything the enemy can use (unless you count vehicles), and a couple of things they can't, but only submachine guns and the Lugers used by German officers can be picked off enemy bodies. That enemy sniper? His rifle disappears when you kill him. And you can't take that guard's grenades, either, or at least not the ones on his model. This is probably to keep you from switching rifles mid-mission or getting too many grenades. Additional weapons must be found in a level, though you can bring any weapon you've found with you in future missions and replays.
Valkyria Chronicles 2 gives the enemy forces a completely separate set of equipment to your own. No matter how many dropped weapons you pick up, you'll have to wait for an Ace to drop the blueprints for an advanced weapon and then reverse-engineer it. Justified in that the rebels are being supplied by the Federation and, being at war, they probably have their home-made weapons chambered for a different round precisely so the enemy can't easily steal them.
Warcraft II: Averted in one level where (as the Alliance) you grab some Orc catapults.
In one of the most glaring cases of Cutscene Power to the Max, one scene has a human footman sneak up on and backstab an orc guarding a catapult, notice a goblin zeppelin flying nearby, and cut the rope on the catapult to take down the zeppelin in a giant fireball (you can't claim units in a normal game, catapults aren't fired by cutting the rope, and, of course, catapults can't hit air units). The credits for Warcraft III had an extended Hilarious Outtakes version of this.
Final Fantasy Tactics is mostly an aversion. You can steal any piece of equipment that an enemy is carrying, unless they have Maintenance, and if a human enemy dies and turns into a treasure box, you can obtain one (but only one) random piece of their equipment by grabbing the box. (If they turn into a crystal instead, then no dice, though you can gain their abilities.)
There was oh so much stuff lying in the background of the Resident Evil series. Most notably, you find a squad of dead soldiers in the sewers of RE2 with MP5s you cannot claim. Also, doesn't it seem odd that none of the hundreds of zombified police officers are carrying their sidearms or ammunition?
Done in a seemingly intentional way to tick you off in Resident Evil Outbreak: You find a dying policeman with a Submachine Gun who will give it to you if you're Kevin (Another cop) or Cindy (Who he mistakes for his girlfriend). If you're anyone else, he just dies with the gun in his lap and no reason is given why you can't just help yourself to it.
How about one of the farm tools that the Ganados were using as weapons. They just seem so much more effective than the knife...
Face it: Wesker's Samurai Edge would've been a nice spoil of war after facing him so many times in 5, though you can make it yours with a healthy application of cheating.
Resident Evil 6 contains an even more frustrating example in Jake/Sherry's campaign. Both work their way out of a laboratory with little more than the hospital garb they're wearing and have to avoid the armed J'avo infesting the place. If you do manage to kill any, though, their guns and machetes dissolve along with them, so you're forced to take them on hand-to-hand until you recover your gear. In a truly "WTF?!" moment atop this, Sherry starts out her scenario with a stun baton— that she took off a dead J'avo!!
Third Person Shooter
In the Crusader games, you can get ammunition, ordnance, medical supplies, money, and other equipment off of dead enemies... but never weapons or shields.
In darkSector, the trope is somewhat averted by being able to pick up the guns, but they blow up in your face after 30 seconds due to 'infection governors'. Why you can't loot the ammo is anybody's guess.
If you watch the blinking of the sensor, most of them are very close to the weapon's magazine.
Averted in Shellshock: Nam `67. You actually can pick up and use enemy guns. On the downside though it means you have to leave your current gun behind.
Wide Open Sandbox
In Assassin's Creed games, neither Alta´r nor Ezio can use the bows archers drop. Brotherhood continues the proud tradition with the new crossbow- and arquesbus-users, though you can loot their ammo for Ezio's use.
On the other hand, you can snatch enemy melee weapons almost at will. While usually their weapons aren't anything special, spears can be thrown and be used to disable large groups of enemies with a singe spin.
Scarface: The World is Yours. A 'boss' in the last level can and will spam you with rockets if you have bad luck. Once you neutralize him, only one rocket is available for use despite the territory you must cover from then on. In the majority of the game, weapons can be looted and tossed in the back of special cars. See how many chainsaws you can collect?
In Minecraft, Zombie Pigmen's gold swords and Skeleton's bows would once never drop upon their deaths. The 1.2 patch made these items Rare Drops, with a chance for these weapons being enchanted.
Non Video Game Examples
A non-videogame example is the Lawgiver from Judge Dredd. The gun is encoded to fire only when its registered user pulls the trigger. Any attempts by anyone else results in the loss of a limb by way of a small explosive charge. Of course, it is possible to override this function in an emergency, as Senior Judges have access to instructions on how to do this.
In Skyfall, Bond has a gun that only works for him, so when a mook picks up said gun just as a komodo dragon approaches, the result is inevitable.
Adventurers! features an inverted example, when Drecker steals a huge sword off an enemy mook. Not the sword the mook was wielding, but another, much bigger and better. The mook complains that he would have used that sword (instead of his usual, which appears to be made of wood) if he'd known about it (likely also a Lampshade Hanging on the Impossible Item Drop).
In Old Man's War, the Colonial Union's troops were equipped with multi-purpose assault rifles that would only fire for somebody with a particular wireless cybernetic "signal".
Dragon Ball Multiverse: The Heliorian Ultra armor only works if synchronized with its rightful owner's brain waves. Will U4 Buu find a way to bypass that security measure?
One of the more interesting Real Life examples was the Russian habit of deliberately building railroads at a different gauge than the rest of Europe. This was to prevent them being used by invaders.
The British used a similar tactic in Sudan: Sudan was theoretically an Anglo-Egyptian co-dominium, but in reality Egypt was a British protectorate—and not a happy one. Since the Sudanese had a history of revolting and the Egyptians would probably be glad to rise against the British if they could get the chance, the British used a narrower gauge in Sudan than in Egypt (whose rail system was older and built to standard gauge).
Averted, however, in the USSR's Winter War and Continuation War with Finland. Both sides used variants of the Mosin-Nagant—Finland had been part of the Russian Empire, after all. While the cartridges used were a little different, they were similar enough that captured Finnish ammunition could be used in the Soviet ones.
In a similar vein, after NATO standardized 80mm mortars, the Soviets designed an 81mm mortar. The Soviet weapon could use captured NATO shells, at reduced efficiency, but the NATO weapon could not safely fire Soviet shells.
Attempts to make this trope Truth in Television with Skyfall-style lockout systems have so far met with failure. Biometrics are still much too expensive to use, and military, police and civilian consumers are reluctant to trust lower-tech options like magnetic or infrared lockouts because of their dodgy reliability.
In the era of flintlock muskets it was often the case that soldiers of the army with the larger calibre could at a pinch use captured enemy cartridges, but the army with the smaller calibre had to rely only on its own ammunition.
The Romans made one of the bolts that fixed the long iron point of the pilum (the legionary's throwing spear) to the wooden shaft of wood so that it would shatter on impact, thus loosening the connection of point and shaft of the pilum and making it impossible for enemies to pick it up and hurl it back at the Romans in battle. The point was also made of easily malleable iron, thus twisting upon piercing your shield and making the shield unusable, as a large piece of iron and a bit of wood was pointing out of it.