Randomly Drops/Playing With

Basic Trope: Items that can be collected from dead creatures don't always appear when you kill them.
 * Straight: There's only a 42% chance of getting a Bear Ass when you kill a bear.
 * Exaggerated: Only 1 in 10 bears will drop a Bear Ass.
 * Downplayed: Only 1 in 10 bears won't drop a Bear Ass.
 * Justified: The merchant specifically needs pristine Bear Asses - any subpar ones are worthless, so your character doesn't even bother picking them up.
 * Being shot in the ass is the Achilles Heel of the feared Iron Bear. Killing one without damaging its ass is a rare and valuable skill, like preparing fugu.
 * The merchant wants bezoars, which are only found in the stomachs of goats - but even then, they're a rarity, and there's no way of telling from outside whether a particular goat has one.
 * Inverted: Each bear you kill drops multiple asses.
 * Subverted: You think Bear Asses are dropping randomly, but once you learn about Subsystem Damage you realise that targeting their heads results in a fat, juicy ass dropping every time.
 * Double Subverted: ???
 * Parodied: The merchant sends you out for Thirty Hens' Teeth, but warns you not to be put off if you don't find any at first because they don't all have them.
 * Zig Zagged: Some items drop every time (Rabbits' Feet), some drop randomly for no apparent reason (Bear Asses), some drop randomly for a good reason (Goat Bezoars), some only drop in specific circumstances (Wolf Pelts must be undamaged) and some drop even if you don't kill the animal (Lizard Tails fall off if you scare them).
 * Averted: Every time you kill an animal, any game-relevant body parts you'd expect them to have will drop.
 * Enforced: Fake Difficulty.
 * Lampshaded: "What, did this bear have no ass? Why are there so many assless bears? No wonder they're valuable..."
 * Invoked: ???
 * Exploited: A Mad Scientist sets up a farm breeding Four-Assed Bears in order to corner the market on the Commonplace Rare commodity.
 * Defied: If you visit a local butcher he promises to teach you the skill of Butchery, which makes all killed creatures drop any body part you might want.
 * Discussed: "I hope all the bears have asses, it's irritating when they don't."
 * Conversed: "So, it's taken me 60 dead bears to find 20 Bear Asses. On the plus side I have 60 of these Bearskin Rugs I can do what I like with..."
 * Played For Laughs: Your sidekick mocks you every time you fail to find an Ass.
 * Played For Drama: You're searching for a Unicorn Horn, and every unicorn you kill pushes you further down the Karma Meter. There is a counterintuitive way of getting a horn without killing any, and this is explained to you at length once you finally deliver the goods.
 * Plotted A Good Waste: There are ever-so-subtle differences between the bears which drop Asses and the ones which don't. Each time you find one, you gain Experience Points in Hunting which make the differences clearer. Hunt accurately enough (or just grind at it enough), and you become a skilled hunter, making future Fetch Quests more lucrative.

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"If you assassinate a bear, one ass in eight will have no hair. A Bare-Assed Bear ass is barely worth a care, but a Hairy-Assed Bear ass is an ass I'd care to bear."