Almost-Lethal Weapons

Weapons will reliably kill the bad guys in one hit, and merely injure a hero, who can hobble off and fully recover by next week.

Definitely not Truth in Television. In Real Life, the chances of you receiving such minor injuries (or no injuries at all) after falling victim to such things are extremely slim, unless you've got some kind of miraculous luck. Almost invariably, you would be either killed, or scarred or crippled for life. Or in the case of ingesting some deadly poison, there's a good chance you'd suffer permanent brain damage. Definitely Don't Try This At Home, folks. As an example, In 1384, Perrin Le Roux was accidentally killed by a wad of paper fired from a cannon (used as a prop in a play) which struck him. The velocity of the projectile, Le Roux standing closer to the cannon than was allowed, and the fact the projectile hit him in the eye led to his death a few days later, in spite of the seemingly innocuous object that struck him.

Even "nonlethal" weapons such as tasers, pepper spray, tear gas, and beanbag guns can kill under the right circumstances, e.g. a taser being used on someone with preexisting health problems or heart disease, or pepper spray or tear gas being used on someone with breathing problems.

Probably a special case of the larger trope of Plot-Sensitive Items, wherein weapons only do as much damage as the plot calls for.

See Also: Improbable Aiming Skills (kill in one shot, even at beyond limit range, or conversely never accidentally strike a mortal blow when not shooting to kill) and A-Team Firing / Imperial Stormtrooper Marksmanship Academy (can't hit the broad side of a mountain at point-blank range) / Made of Iron (Human Beings Without Special Powers surviving things they really, really shouldn't))

Anime

 * In the first Appleseed movie the cyborg assassins sent after Deunan ues their monomolecular whips on anything except living beings. They go to hand to hand combat with Briareos, a practical human tank, and get punched into pieces for their troubles, instead of doing the reasonable thing and slicing him up from the distance.

Film

 * Codified by John Woo's A Better Tomorrow 2. In the final scene, approximately 80 mooks are killed with every weapon imaginable. The heroes also suffer injuries, but appear to suffer no ill effects. At the end, they calmly sit in their blood-soaked clothes and wait for the cops to arrive. Interestingly, the only John Woo movies where the heroes suffer the same injuries as the villains are A Better Tomorrow (1) and, perhaps, The Killer.
 * The Spider Man series of films doesn't know how lethal the damn pumpkin bombs are. In the first movie, they vaporized anything they came in contact with. In the third movie, pretty much they just ended up knocking people down - even when a villain not wearing any armor takes a direct hit! Then, at the end of the film, gets hit by one and is again, vaporized.
 * Partially justified by the possibility of different kinds of pumpkin bombs - the vaporization in the first film was definitely not a regular explosion.
 * Also,
 * In Star Wars, when Han shoots Greedo with a blaster, there is a small explosion, turning Greedo into a blackened, smoking corpse. In Return of the Jedi, Leia takes a blaster hit in the shoulder with only a minor injury.
 * Range could be a factor; Han Shot First at point blank range, while Leia was basically sniped by a Stormtrooper.
 * As could size. Han's weapon seems more powerful that the tiny holdout weapon that the stormtrooper used on Leia. It's like shooting someone from 100 feet with a .22 rimfire. Yeah, it's gonna sting, but you can get up in a bit. Still follows the trope, though.
 * In the Star Wars Universe, Hans pistol, the DL-44, is one of the most powerful blaster pistols in the universe. Assuming the holdout weapon the stormtrooper used on Leia was a scout blaster, comparing the two would be similar to comparing a .44 Magnum to a .38 SPL. Also, their different goals: despite their names, the stormtroopers believed in taking prisoners, not disintegrating on sight. These troopers have "recapture alive" as a priority, along with "don't punch holes in the bulkheads we can't repair", so they'd be issued less-powerful weapons. On the other hand, Han is a smuggler who has only one thing he wants to do to any bounty hunter or pirate on his case: KILL. Thus he buys the most powerful blaster he can get in pistol size, and leaves it dialed up to "incinerate". While this may semi-justify this trope, this troper is amazed the stormtrooper actually hit a major character considering the dominant trope in the series.
 * If one applies the gun mechanics from the games to the films, this is justified. Many blasters have different power levels, and heavy blaster pistols like Han's, in particular, can charge up to five charges into a single shot; this is, more often than not, enough for a single-hit-kill. In the movie, one would think Han had plenty of time to charge the shot while talking to Greedo - or maybe he holds a shot pre-charged.
 * Careful observation of where the blaster bolt hits shows it strikes the wall beside Leia. She was hit by the splash damage from the blaster bolt's impact and shrapnel from the wall.
 * Darth Vader stops Han's blaster with one hand, using the Force. The EU offers a couple of explanations, such as a Force power that allowed him to absorb energy on contact and use it to power other Force abilities, and the Knights of the Old Republic game has Force Deflection, which allows the character in question to deflect blaster bolts with their bare hands.

Live Action TV

 * Stargate SG-1, where Jaffa keel over from one hit but the team take multiple hits from the same weapons over the course of the series. It should be noted that three of SG-1 have been killed by staff weapons (and were brought back by Sufficiently Advanced Aliens).
 * One episode hung a lampshade on it by having someone read the long list of injuries O'Neill suffered over the course of his career with the express purpose of pointing out how unusual that was of him.
 * Strangely, Jaffas are supposed to be tougher than humans...
 * However, this could be justified by the fact that the Jaffa rarely do any healing (as in, clean a wound, bandage it, bedrest etc.). Bullets are repeatedly shown to be much more deadly than the staff weapons. When someone in the SGC is hit, they are given first aid and are transported to the infirmary as soon as possible. Apparently even though Jaffa are tougher, they can be killed by a few bullets just as easily as an ordinary human - most of their "toughness" is more about strength (useless in ranged fight) and resistance to illness (would help recovery but is pretty much useless if you bleed out in just a few minutes).
 * Supernatural: While the Winchester boys's various weapons work pretty reliably on the bad guys, Sam and Dean themselves have managed to survive car crashes, bullet wounds, head trauma and strangulation (among many, many other mishaps) with, at worst, a broken bone.
 * Tended to happen on Star Trek, especially Voyager and Enterprise. A single phaser shot would kill an enemy mook no problem, but anyone with their name in the credits rarely suffered more than shoulder and leg hits that were completely cured. If you didn't have your name in the credits, you usually aren't so lucky, particularly if you happen to be wearing a gold or red shirt at the time. Nog learn this the hard way, when he lost a leg.
 * Quite a few episodes have main characters taking a phaser (or whatever energy weapons) hit at center of mass in the chest, point blank range only to be barely inconvenienced (at most they'll be knocked out for a bit or have to limp).
 * One of the best examples was in a Voyager episode. A nameless Gold-shirt was hit in the shoulder by a small pistol and instantly died, proving the weapons weren't on stun. Chakotay took a blast from a large rifle directly to his center mass... and woke up with a headache. It did do some nerve damage however that unless treated could eventually kill him, but the fact it didn't kill him outright is rather ridiculous.
 * Another example is during the TNG episode "Enemy Mine", where Geordie and an annoying Starfleet officer get hit with the same weapon, seconds apart, in the exact same way. Next scene, Geordie's nursing a headache from the blast, while they're covering the other guy with a sheet.
 * Justified in most cases by the stun setting.
 * In Battlestar Galactica Reimagined, shoots   twice in the chest at close range with her side arm, and he survives. Needed a long stay in the sick bay, though. Later, she shoots   in the chest at short range with her side arm, and she dies within a minute.

Tabletop Games

 * The Tabletop Game Feng Shui has this effect for all named characters, heroes and villains alike, reflecting how tough major characters in Heroic Bloodshed movies tend to be in regards to bullets.
 * Invoked in the D&D 3rd edition player's handbook as an explanation for why characters gain additional hit points as they grow in level, as well as for why they recover them faster. A 1d8 longsword will usually fatally injure a peasant, but the same attack results in an entirely superficial injury if used against a high-level fighter.
 * D&D 4th Edition has "minions", enemies with a single HP. In nearly all other regards, they are normal foes.
 * Well almost. They deal flat damage - and little damage at that (meaning they don't benefit from critical hits) and have less powers than most monsters. Also few minions rarely have ranged attacks and almost never area attacks. They can still be a threat because they are present in huge numbers, allowing them to deal Death of a Thousand Cuts on PCs, as well as serve as a distraction when fighting alongside bigger threats.
 * Mutants and Masterminds has had this mechanic from the beginning though its better defined in 2nd edition. Pretty much everyone aside from the important characters (good or bad) are knocked out after one good hit. In both this and the above case, they're deliberately mirroring the presence of this trope in their respective genres.

Video Games

 * Pick a First-Person Shooter. Any First-Person Shooter. Arguably this is for gameplay reasons, though.
 * However, higher difficulties in these games avoid the use of this trope, allowing the player character to be killed by single (or very few) shots. Also, more 'realistic' tactical shooters such as Rainbow Six and their ilk feature very fragile (by FPS standards) player characters, with injuries commonly sidelining characters for succeeding missions.
 * Played straight and subverted by the Max Payne video game series. In terms of gameplay, Max actually has fewer hitpoints than some of his later enemies, though he can regenerate with painkillers. Viewed in terms of realism, Max is peppered full of holes from desert eagles, assault rifles, and grenade shrapnel for several hours of gameplay and needs nothing more than aspirin to stay healthy.
 * Starting with the third "main story" game in the Wing Commander video game series, the player's fighter gets extra damage absorption ability, compared to the same fighter flown by AI pilots, either friend or foe. In an extreme abuse of this property, if you and the enemy are flying the same ship, as in the final flight mission of Wing Commander IV, you can contrive a situation where the enemy runs into you at full speed, killing the enemy while leaving your ship significantly damaged but surviving for auto-repair to kick in.
 * Justified in the Halo games. The best-equipped soldiers, both human and Covenant, have plasma shields. These prevent instant death most of the time, but a handful of weapons are still one-hit kills if properly aimed. In other cases -- especially with the plasma pistol -- your shields may be downed in a single shot, at which point you can be killed with realistic ease, like any Mook.
 * Or if you play on Heroic (the way the games are meant to be played) or Legendary (which is what the books assume), in which case it's completely averted.
 * Indecisively used on sniper rifles, which will One-Hit Kill you on a head shot but just take out your shield if aimed anywhere else. Either sniper rifles magically do more damage when aimed at a head, or your Force Field shield is weakest in the place it should be strongest.
 * Brutally averted in First Encounter Assault Recon - unless your armor and health are maxed out, if the enemy shoots you with a weapon that instagibs them, you will die instantly in turn.
 * In Ace Attorney, murder victims rarely require more than a single blow, stab, or shot to die. But if you're a major character like, a bullet is a mere inconvenience.