Selectively-Lethal Weapon

Sometimes a character has one weapon that, regardless of hows it's used and how improbable it would be, can be either lethal or nonlethal. They throw one, and it cuts through a steel cable; next throw, it knocks someone out without even drawing blood. This is the Selectively Lethal Weapon, a weapon that should be either always lethal, or always nonlethal, but somehow manages to be a bit of both, usually with an extremely tenuous explanation (if one is provided at all).

Contrast with Stun Guns, which are something a bit different.

Anime and Manga

 * In One Piece, Zoro's sensei demonstrates the ability to leave paper intact with his sword slices.

Comic Books

 * Batarangs qualify - at times they're sharp enough to cut steel, and other times they only knock you out. Occasionally, they blow up in your face. This being Batman, he usually explicitly has more than one type on hand.
 * A diagram in one book showed that the front "round part" is hard and blunt, perfect for knocking thugs out. The back part with all the spiky bits is razor sharp. Batman can throw it so that whichever end he wants strikes the target.

Literature

 * The titular sword of the Sword of Truth series works this way - the sword can only cut if the wielder believes the target is an enemy . The wizard who gave Richard the sword stresses that the user doesn't have to be right in their beliefs, they merely have to believe... which is why wizards agonize over who gets the title that the sword goes with.

Live-Action TV

 * Xena: Warrior Princess has her Chakram: it often went from blunt enough to KO gangs of mooks to being able to slice through armor... occasionally in the same episode. Occasionally on the same throw.
 * Star Trek phasers justify this with having multiple intensities, including a stun setting (you could probably also call this trope "Had it Set to Stun"). Some have noticed a tendency, regardless of other circumstances, for the lethality of phasers to be inversely proportional to the importance of the character they're being fired at.

Tabletop Games

 * Dungeons and Dragons has merciful weapons that are enchanted to deal an extra 1d6 damage, but have all the damage nonlethal. This ability can be switched on and off as a free action, and KO'd enemies can be killed at your leisure, so there's not even a drawback. Other weapons can be used to deal nonlethal damage, but at a penalty to accuracy. And just to round out the trope, normally nonlethal weapons can take this same accuracy penalty to deal lethal damage.
 * In the 4th Edition you can declare any attack to be retroactively non-lethal, up to and including a disintegrate spell.
 * In Werewolf: The Apocalypse, the Children of Gaia, a tribe of pacifists, have a spell that somehow turns their teeth and claws into nonlethal damage, allowing them to take down enemies with full force but avoid killing them.

Video Games

 * The Medic Crossbow in Team Fortress 2 heals allies and damages enemies, which makes it hilarious to fire into the melee on the 3rd point in Medieval Mode.
 * The Batman Arkham games give you multiple types of Batarang as early as Arkham Asylum; one of them can short circuit the electronic collars some of the goons are wearing.
 * NetHack invokes this (a bit counter-intuitively) with nurses - they will hit and damage you like any other enemy unless you're disrobed and are not wielding anything... at which point they still attack, but every "hit" heals you. Using #chat on one gives you hints to that end, but you're not likely to render yourself armorless for no good reason...
 * ...at least until you've made sure it's safe first.

Western Animation

 * Sokka's boomerang in Avatar: The Last Airbender is similar to the Batman example. While it is used to cut and knock out, Sokka is often shown to be sharpening one edge of it while leaving the other edge dull. It can be assumed he chooses which side to strike with.
 * Samurai Jack: The magical sword the title character wields will not harm those pure of heart - on the occasion where it was used against him, it merely bounced off, and when stolen and used against others, the best it can do is knock people away.
 * Adventures of the Galaxy Rangers did a nice dodge on this trope. Early on, Zachary would sometimes explicitly order his team to "set blasters for stun." Later in the series, when they were trying to score a toy deal, they had a crook say that Ranger blasters didn't have a kill setting. Now, seeing as the source of the information was a very dumb crook, the writers could both have the kid-friendly "stun only" mention on camera and a wink to the more likely prospect that there was a kill setting. Likewise, Crown blasters also had stun and kill settings, but it was justified in that the Queen wanted humans (or other compatible species) as fodder for the Psychocrypt, and you couldn't drain Life Energy from dead enemies.