NetHack/YMMV

"Core dumped."
 * Breather Boss:
 * Optional Boss Croesus is actually a pretty tough melee fighter and fast to boot, but is much easier compared to the quest nemeses and other bosses, and can easily be dispatched through various 'standard' methods.
 * The Wizard quest nemesis, The Dark One, is a foe that primarily relies on spellcasting - against a player character whose role will always have at least one source of magic resistance, barring unfortunate circumstances. Even better, as of 3.6.0 the Eye must be worn to actually convey magic resistance; though he has a high monster magic resistance and decent AC, this means certain spells and wands can still make short work of him. Even in 3.4.3, where the Eye merely had to be carried, the worst his magic resistance did was slightly narrow a player's options.
 * Similarly, Thoth Amon, the Barbarian Quest nemesis, can be rendered trivial with a magic resistance. Unlike the Dark One, he isn't facing off against a player that's guaranteed to have it - unfortunately for Thoth Amon, he has far lower monster MR, meaning even a Barbarian that doesn't have magic resistance handy can still trump him with a wand of cancellation or (better yet) a wand of death!
 * Broken Base: The major arguments in fandom so far have been:
 * Should the game be played using the graphical tile-sets or the original ASCII format?
 * Is it a legitimate tactic to exploit programming quirks which allow such things as "pudding farming" (see below)?
 * The changes to the protective ward "Elbereth" in 3.6.0 were also a significant Base Breaker, to the point that 3.6.1 walked a few of them back.
 * Demonic Spiders: There's at least two or three at any point:
 * On early levels, you have to deal with floating eyes which paralyze you, gas spores which do obscene damage by exploding when they die, and killer bees that can poison and instakill you. Floating eyes can be avoided, and killer bees respect Elbereth, but may The Lady help you when your overleveled pet decides to start attacking that gas spore right next to you...
 * On early-middling levels, you have to deal with monstrous ants, including soldier ants -- the most common enemy-based cause of death in the game (accounting for 1.75% of all deaths on nethack.alt.org). Go team ant!
 * On the middling levels, you have the infamous cockatrices and many enemies who can swallow you and kill you, including lurkers and purple worms (which can also be encountered on the earlier levels).
 * Then you must contend with demon lords and princes, and should you actually survive them and get the Amulet of Yendor, you must face the Wizard of Yendor.
 * Excuse Plot: There is an amulet in this dungeon. You need to find it, then return it to your god. Go.
 * The details and quest branch vary quite widely with each class, but ultimately the basic plot progression is the same- essentially by design, given the amount of randomized features. As reliance on a fixed script would ultimately hinder things, this allows the player to organically "develop" their own story simply by playing through the game.
 * Fridge Horror: There are lots of "used armor" shops scattered throughout the dungeons. Sometimes these shops contain cursed armor, which can't be removed, except by uncursing it or if the original owner dies wearing it. The horror comes when you realize where the shopkeeper gets his inventory.
 * And if you die in a shop, the game flat-out tells you that the shopkeeper takes all your possessions. If you somehow directly steal from the shop (e.g. teleporting out with unpaid items, digging out...), the shop owner will chase you. If or when you die after, guess who comes for the loot?
 * Game Breaker:
 * A very controversial thing to do in Nethack pre-3.6.0 is "pudding farming", causing an enemy that splits into two whenever you attack it to split multiple times to abuse the game's prayer and sacrifice systems, as well as farm items. While it does render the game ridiculously easy, it also turns the whole affair into a tedious slog - and nearly anything you'd accomplish with pudding farming won't help you as much on The joke goes that the DevTeam let this go unsolved for so long because they'd already implemented an immediate and savage punishment for pudding farmers.
 * In a similar vein, players can repeatedly kill ; as he reincarnates an infinite number of times, but gives full points for each kill, hitting the max score is trivial with the proper setup. Subverted in that it's generally considered a sign of skill to ascend with a lower score rather than a higher one (e.g. by doing a Pacifist Run).
 * Genius Bonus: Many things in Nethack, including some of its Shout-Outs, are very subtle, while others are more surprisingly (and distressingly) logical; the phrase 'NetHack is not real life" was coined in response to common suggestions for new idea that suggest items and mechanics in-game act more "realistically".
 * An enemy named the "quantum mechanic" sometimes carries a box that it drops upon death. Inside the box is a cat named Schrodinger's Cat, which has a 50/50 chance of being either alive or dead. If you examine the game's source code, you will learn that the state of the cat is not determined until you open the box.
 * Some fantasy items benefit you if you know the myths without even having read a spoiler:
 * Since the game has strong Unix origins, there's also plenty of jokes only a Unix/Linux geek would understand.


 * Goddamned Bats: Nymphs, floating eyes, actual bats, and leprechauns, to name but a few.
 * The Scrappy: The Caveman was one of the more maligned classes for a long while, believed to be simply a weaker version of the Barbarian and Valkyrie with few of their relative advantages and more restrictive weapon choices - this often locked them out of most common artifact wishes, and any artifact whose weapon skill they could unrestrict would be capped at Basic. That said, their high strength and constitution still make them a strong choice, and the ability to multishot spears and upgrades to weapon options such as the aklys in 3.6.1 helped mend their reputation.
 * Scrappy Weapon:
 * Many weapons in the game end up underused due to the presence of artifacts, but slings in particular are considered junky for even most starting characters - as such, they are only used as a last resort alternative to not having any form of ranged attack at all, due to rocks (the most basic form of sling ammo) being plentiful and easy to gather.
 * Among artifacts, the Weapon of X-Slaying "Banes" are usually considered too niche to main as weapons on their own, as there are a wide variety of foes to deal with in the late game - though there are naturally some exceptions, and at minimum, most of them can serve as a gateway to training for more desired artifacts. Trollsbane is considered the weakest of the sacrifice gift artifacts even by the standards of the other Banes.
 * Trolls can be especially troublesome, but outside of perhaps throne rooms and the Barbarian quest, an average player is not likely to run into comparatively many trolls to begin with. Furthermore, the base item is a morning star, which the player won't have much incentive to train where other 'artifact base' weapons like long swords are much more available. Finally, Trollsbane can prevent corpses from reviving, but that function can also be fulfilled through other means (e.g. having a carnivorous pet eat the corpse).
 * Among quest artifacts, The Heart of Ahriman is considered one of the weakest - not because it doesn't offer anything worthwhile, but because what it does offer will likely be painfully redundant to a quest-ready Barbarian.
 * The base item, a luckstone, is useful but can be found far earlier, with one guaranteed at the end of the Gnomish Mines; the Heart also grants stealth when carried, but Barbarians gain intrinsic stealth at level 15, and every quest requires the player to be at least level 14 to proceed. That said, the Heart still does have unambiguous benefits - it is an intelligent artifact, meaning that it can resist the "curse items" monster spell 4/5 of the time. (And no one wants to carry around a cursed luckstone!) It can also be invoked for levitation - while there are multiple sources potentially available by the time you get the artifact, its levitation uniquely extends to any steeds you're riding as well. The use of bullwhips to retrieve items that a levitating character normally can't reach is subjected to dexterity checks, a stat Barbarians naturally excel in.
 * Similarly, the Longbow of Diana is considered to be relatively unpopular for Rangers for multiple reasons - some of which overlap with issues tied to the Ranger quest's home level itself. The long and short of it is that it provides reflection when wielded, telepathy when carried, and can be invoked for a source of effectively infinite arrows - all decent qualities in themselves. The problem? The entire quest is infested with centaurs who often carry attack wands and will zap them from places where your arrows will have trouble reaching, which will almost necessitate having reflection just to enter!
 * Its weapon type is just a regular bow, which means non-human Rangers are cheated out of 1 point of racial multishot bonus if they planned to make extensive use of the Longbow. To add insult to injury, most of the arrows it generates will likely be unenchanted (i.e. +0), which is a slight problem if you're trying to manage your arrow stacks.
 * "Stop Having Fun!" Guys: Using any movement key configuration other than HJKLYUBN (or maybe numpad) used to be considered Serious Business, but isn't harped on as much nowadays.
 * That One Boss:
 * Master Kaen, the Monk quest nemesis, ended up as this due to the Monk role being imported from SLASH'EM, where Monks had special techniques that allowed them to match him in terms of damage output. Without those techniques, the Monk is left to contend against Kaen's monstrous melee damage through other means.
 * Among Gehennom's Optional Bosses, doubtlessly qualifies, having the highest monster difficulty rating in the entire game.
 * The Wizard of Yendor can count as this, due to his propensity for coming back with more hit points and a higher caster level than before.
 * That One Level: Gehennom is actually 20 to 24 consecutive Scrappy Levels. One of the many additions SLASH'EM makes is breaking up the monotony of Gehennom with more "special" levels, while Sporkhack completely revamps Gehennom by (a) giving the local Mooks some ability to actually threaten the player (b) getting rid of the mazes in favour of caves (with lava behind the cave walls).
 * Unfortunate Implications: The foocubi and their seduction attacks can easily read as sexual assault or outright rape to many players, and patching in ways to avert this have been a notable subject of discussion. For what it's worth, the game has a compile-time option SEDUCE; if not defined, the foocubi's seduction attack becomes a "drain life" attack.