Dwarf Fortress/YMMV

Also in previous versions, a Forgotten Beast made of poison mist. Then, a forgotten beast made of any intangible material could only be killed by being encased on obsidian or ice. However, forgotten beasts made of such material are now laughably easy to kill; one punch will tear off limbs. "Today: Then I talked to one of [the resurrected severed limbs], and it told me that it was peasant. It was flattered but had no need of my services. I imagine its little fingers were shaped into the form of a mouth and they flapped back and forth while it spoke with a high-pitched voice. I guess there's still work to do."
 * And the Fandom Rejoiced: Several new additions, but the introduction of the curse system immediately comes to mind.
 * "Added ability to make designations over Z levels." Goodbye, Scrappy Mechanic!
 * The addition of Adventurer interaction, including necromancy
 * The new changes to evil biomes turned them from mildy harder than normal to almost impossible, due to deadly weather and corpses refusing to stay down, even after dismemberment. This being the Dwarf Fortress fandom, they were ecstatic with this new source of Fun.
 * The release of version 34.06, which among other things, fixed the clothing bug and completely revamped the animal training system.
 * The announcement of major hauling fixes, which includes the addition of minecarts.
 * Broken Base: The friendly teasing that occurs between tile users and ASCII users, or between those who prefer Adventure Mode to Fortress Mode, is occasionally mistaken for this.
 * Players that find Fan Nicknames fun and those who don't have been a straight example at times.
 * And now you should see what happened with the tweaks to drop rate. The !!debates!! are rapidly becoming legendary.
 * Demonic Spiders: The giant, venomous, web spitting kind. Also Wolves, especially in Adventure Mode, where they can ambush a hapless low-level traveler and kill them in a matter of moments, provided they're surrounded.
 * If you ever see a zombie giant cave spider, RUN. RUN AND NEVER FUCKING LOOK BACK.
 * Elephants have been toned down greatly from the days of Boatmurdered, but unicorns have since gained a reputation as psychotic serial killers who brutally murder dwarves at random.
 * In previous versions there were the elephants, then the carp (even Toady One thought he made them too strong). Now it's Giant Badgers. You also occasionally run into randomly generated enemies that are made of solid stone or even metal, and are as hard to kill as that would imply. To make things even worse, if the Random Number God hates you, they can also have a dust attack that in its most deadly form an Instant Death Radius with about the same range as your archers.
 * Although they're not strictly enemies, they deserve mention here: The evil biomes occasionally have clouds of ash or mist that wash across the map, and, just like a forgotten beast, poison your dwarves with a random syndrome. A few minutes after the new version was released there already were reports of particularly nasty clouds wiping out entire embarks before they had a chance to dig in. The worst of these will zombify your dwarves instantly and turn them against you. Your best bet in evil biomes is to seal yourself off from the outside world and never look back.
 * Particularly evil biomes have spontaneously rising corpses, which can easily fall into this trope both on their own, and as a result of the fact that all their severed limbs also rise up and create more enemies. The only certain way to kill them is, of course, by dismembering them, then crushing the remains under a drawbridge before they reanimate. Not even magma can kill zombies; it just makes them angrier and on fire.
 * The "husks" produced by the aforementioned clouds are themselves this trope - even more so than the endless rising body parts. Not only are they insanely strong and near-impossible to kill, they tend to be covered in the dust that zombified them... which means that almost anything that touches it will become a husk itself. One thread on the forums mentions the results of You can start defecating uncontrollably in sheer terror now.
 * Giant mosquitoes in the intial 2012 release deserve a mention here, as, even though they're no match for a trained dwarf individually, every time they show up they bring a hundred friends with them, and, even if you simply lock yourself underground to avoid them, they'll lag your game to high hell due to sheer numbers. Luckily, this was because of a bug that was fixed within a few days.
 * Giant sponges have joined the ranks of carp with the new release. They're not supposed to move, so you can't drive them away from your fort, but somehow they can easily crush your dwarves by pushing them around (they're huge, so even a push can crush bones), and are immune to normal weaponry due to lack of body parts and blood. Also, they can spawn in rivers and lakes, too. Normal sponges are like this, but without the killer strength and they can spawn anywhere that has water (giant sponges only live in savage areas).
 * Ear Worm: the background music, a rather hypnotic six-string guitar piece by the game programmer himself.
 * Its ability to get stuck in your head combined with the listener's inability to actually remember how it goes suggests it may be some kind of ethereal god music.
 * Fan Nickname: "Dorfs" for dwarves, "Dorf Fortress", "Urist McX" for any given dwarf, "Cutebolds" for kobolds, "Hidden Fun Stuff" for gateways to hell.
 * "Cotton Candy" for adamantine, "Clowns" for demons, and "The circus" for hell.
 * "Party People" for.
 * "Goblinite" or "Goblin Christmas" for the vast bounty of meltable iron weapons and armors a Goblin Siege would bring, once your forces had killed them. No longer the case as of DF2010, however, as they now use mostly leather, and frequently use lesser metals for their weapons and helmets.
 * "Zombie Spirals" for a common occurrence when the player embarks in an evil biome that raises everything as undead. The more that get killed, the more undead there are to fight, until your dwarves are completely overwhelmed.
 * Game Breaker: Arrows and thrown objects.
 * Ballistas. There have been reports of people holding off huge raids with just one ballista, and people going on rampages in Adventure Mode with just a ballista bolt.
 * Whips, flails (blunt) and scourges (edged) have an extremely tiny area of impact that offsets the fact that they have a relatively weak attack overall. Compared to other weapon attacks, a successful hit by one of these will focus all the weapon's power (and weight) into a single point on the target rather than a wide area. This will nearly always instantly crush (whip, flail) or sever (scourge) a body part, making them the best melee weapons in the game by far.
 * Fan Nickname: Dwarven lightsabers/hypersonic hammers(flails).
 * Weapon Traps are considered to be game breakers by some due to the massive amount of damage they deal. Likewise for cage traps, which unfailingly captures anything that isn't immune to traps, even megabeasts (and if you put a Giant Cave Spider web on it, it catches anything)
 * Economically, stoneware pottery. Fire clay can be gathered indefinitely and stockpiled near a magma kiln, which perpetually churns out crafts and large pots. Considering that stoneware can hold liquid without being glazed, you will never have problems with booze again. The value of stoneware crafts is comparable to obsidian, but are much easier to mass-produce; a few years of constant production will give incredible purchasing power.
 * In Adventurer mode, sneak and find a tile where rocks can be picked up that is as close as you can get to the enemy's range of vision. Then start throwing the rocks until he dies (or passes out, at which point you can walk up and slit his throat), without ever being seen. A higher Ambusher skill makes this better, of course.
 * Make a small room full of low-quality menacing spike traps. Fill those traps with practice spears. Attach them to a lever or repeater. Station a squad of dorfs in the room and watch their dodging, blocking, armor using, shield using, and weapon skills go up at a ridiculous rate. As a bonus, if any of them actually manage to get hurt, it gives your doctors practice, too!
 * Goddamned Bats: Bogeymen. The only thing stopping them from being Demonic Spiders is that a decently skilled/strong character can usually manage to kill them; they're still absolute bastards, though, since you can almost never fucking hit them.
 * Good Bad Bugs: These are the kinds of bugs that make DF the game it is. Magma sea vanishing? Oops, it's all being.
 * Due to a few of the particulars of combat and skill training, along with an overpowered bite attack, carp in previous versions had the ability to very quickly tear your people to shreds.
 * A good bit of the popularity of the game -- aside from the whole ridiculously-detailed fortress-building thing -- is due to various bugs, perhaps "misfeatures", that produce unintentionally hilarious results: for example, dwarves not recognizing that they are on fire before attempting to drink from the extremely flammable booze stocks. The developer's constantly updated progress log is a good source for these stories.
 * See the community page for a batch of examples that the playerbase named.
 * Pinching (yes, pinching) is ludicrously overpowered. Pinching someone in the neck while they're sleeping will sever their head and send it flying across the room.
 * And of course, the nicely illustrative, 'Got rid of world gen crash during succession after death of prolific long-standing position holders with inbred descendants.'
 * A bug in the way cooking ingredients used to work, which enabled a cook to produce solid meals out of nothing but booze; this has since been fixed.
 * Due to a rather peculiar glitch that has since been fixed, ordering the construction of metal goblets would turn even the most valuable bars into iron mugs. Some reported that iron bars turned into gold, as something of an alchemical counterbalance.
 * Let's not forget the fact that you can throw things without having usable arms. Or legs for that matter. Apparently your character spits them out.
 * For the first few 2010 versions, it was possible for creatures, including your dwarves, to melt if caught in the rain in a warm area.
 * This leads to one of the most ridiculous exploits ever in Adventurer mode: by going in and out of fire to melt all the fat in your body (but avoiding bleeding to death in the process) you become effectively fireproof.
 * Speaking of which, the Adventure mode also has several conditions where your controlled character suddenly shifts to some other creature. Overlaps with Game Breaking Bugs depending on luck and personal preference as you may get something stupid like a kobold or something horrendously awesome and nigh-unkillable like a bronze colossus.
 * More in adventure mode: the effects of walking with a crutch (speed becomes dependent on crutch-walking skill, which grows as you move) only cease when you drop a crutch, not when you put it in a container or throw it. This lets adventurers missing legs regain mobility without losing use of a limb in the process and sell back crutches they're already "done" with.
 * Water will freeze in cold environments, the resulting ice can be mined out like stone, it will melt if it gets too warm. This is all logical and expected. But constructions built out of ice will never melt. You can build a magma aqueduct out of ice if you so desire.
 * Normally, necromancers and other unnatural types make people around them increasingly suspicious with their agelessness until they're run out of town. So, from the devlog: "In bug news, the zombies in a necromancer's tower became suspicious after the necromancer failed to age, and he fled into the hills."
 * More necromancy hilarity, as we see the very silly results when certain abilities are not tied to specific body parts:


 * Dwarfs ordered to remain inside (40d) or in a burrow (2010 and newer) will exhibit a few bugs. A moody dwarf will ignore this order when deciding on what workshop to take, and will bring items to said workshop even if it's outdoors/out of the burrow. One still-standing glitch causes a moody dwarf to forget whether or not he brought items to a workshop that's outside of the area he's supposed to be in. Cue the creation of Planepacked, which took over a year for all the materials that ended up being used to be gathered and contained  73 images of itself.
 * A bug that briefly appeared in at least one version caused a sort of Goblin Civil War. Somehow, goblins were divided on whether or not they were loyal to Non-goblin leaders of goblin civilizations. This caused Goblin ambushes and sieges to immediately start killing each other as soon as they arrived on the map.
 * In one version of Adventure mode, sleeping on the beach may result in you being ambushed... by fish that immediately drown as soon as the battle starts.
 * Hilarity Ensues: Look, if you actually get upset when one of your dwarves gets into a foul mood because you killed his cat on accident, beats up another dwarf who then gets ticked off enough to put his pick into the head of another dwarf who then lies there decaying on the ground, causing bad smells that drive a handful of the other dwarves unhappy enough to pick up axes until bleeding and/or insane and/or dead dwarves litter your fortress, you're playing it wrong.
 * Losing is fun!
 * Memetic Badass: Toady himself. When you look at what he created, it's not all that surprising.
 * Any fortress will create a few of these if it survives long enough, but some are truly exceptional, such as the backpack-wielding dwarf of Syrupleaf, or the dwarves of Bronzemurder.
 * Captain Ironblood who never bathes (and is thus literally covered in blood, mud and vomit at all times) and can kill titans and dragons on his own. He eventually took up seige weaponry use as a hobby.
 * Cacame Awemedinidae, The Immortal Onslaught, Elf King of the Dwarves. According to the (partially player-invented story), he joined the Dwarven military after his wife was killed and eaten by other Elves, just so that he could kill more elves. He proved to hate elves so much, that he was made their king.
 * Memetic Mutation: "Person cancels Action, interrupted by Thing", "Person cancels Action, taken by mood", and "Person cancels Action, too insane".
 * "I think I made fish too hardcore."
 * This is an artifact page. All craftmanship is of highest quality. It menaces with spikes of trope.
 * Losing is fun.
 * Despite not having any real preference for clothing, it's become common wisdom that Dwarves will go through Hell (literally) for a really nice sock.
 * Moral Event Horizon: Mermaid farming upset Toady so much he nerfed the value of mermaid-based materials in the next update so it would stop happening.
 * Ruined FOREVER: A lot of people hated the new, much more complex and detailed military system released with .31. Partly this was due to balancing and bugs, and some of the hate has diminished since initial release as they got fixed. However, 13 point-releases later, even the people who were enthusiastic for it from the beginning are still discovering many subtle inner workings and finding new best practices for creating epic military forces.
 * The release of .31.13 introduced a mind-boggling amount of change to civilization development, including city sprawl, farms, villages, trade, the possibility of literally hundreds of thousands of unique individuals tracked through wars and city building through a thousand years of world generation. At default settings as much as 10% of the map can end up covered with city/village/farmland sprawl. On the one hand it puts a thousand years of history of civilization and development into a slightly more realistic perspective. On the other, it leaves much less "wilderness" and unpopulated land suitable for new fortresses. And on the gripping hand, Adventure Mode now has orders of magnitude more interesting landscape and people with which to interact. Let's just say the changes have caused much debate. However, if history is any indication, balancing and tweaking will occur, and everyone will be happy again. (Except the dwarves. They are all still doomed.)
 * Rule Of Fun
 * The Scrappy: Everyone hates the elves. They're irritating, stuck-up little bastards who spend all their time getting snooty at you for chopping down trees or selling them wood and trying to sell you poor-quality trade goods (made of wood). Elves are basically the reason people invented trade-depot-drowning traps.
 * Not everyone hates Elves. They are very useful; they come every year, bringing exotic animals and vital supplies, along with valuable stacks of cloth. And, when you seize all their belongings for free and send them back into the wilds, they come back the next year with even better stuff!
 * Players also devote a lot of time and energy devising "unfortunate accidents" to befall their nobles.
 * Scrappy Mechanic: The economy in 40d. There's a widely recommended option to turn it off, and several other options to turn it down if you do permit it (none of which allow you to convert a stack of coins from a blindingly spammy list of one-coin objects.) DF2010 just scrapped the whole thing until Toady can make something that isn't horrendously broken.
 * The inability to designate staircases, item properties, or tree clearing across vertical levels, although this has been fixed as of the February 14 release.
 * Sequel Displacement: Slaves to Armok Chapter 1? Wha?
 * The Woobie: Again, kobolds.