These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Ancient Domains Of Mystery
Demonic Spiders: Including literal spiders, who get summoned in hordes and have poison attacks; jackalweres, who summon legions of increasingly-powerful jackals, the greater claw bugs and killer bugs in the Bug Temple, and pretty much everything you meet in the Final Dungeon.
Ghosts can pass through walls and have an aging touch, making them this to any player race with a short lifespan. A "tension room" full of ghosts can easily surround an orc or troll in a corridor and age him to death.
Ogre magi are at least as tough and strong as normal ogres, regenerate their wounds, and can cast invisibility on themselves and ice spells at the player.
Dark elven priestesses, princesses and wizards (called spider factories) are particularly nasty summoners. Not only they summon hordes of spiders, which fill the area with immobilising webs and use poison, but have themselves nasty attacks such as paralyzation and energy ray spells.
Goddamn Bats: Summoners of any kind can quickly become unbearable. Karmic creatures are just as bad, because hitting them causes your character to get cursed and/or doomed, which is a highly undesirable status effect. And also cats. Killing even one of them makes an enemy of a high-level NPC in the end game instead of a very valuable artifact ring.
Good Bad Bug: Let a dragon pick up a pile of gold. Kill the dragon. Notice that it gives you twice as much, every time. Lots of fun with vaults, although you've got to be careful if the dragons have a breath weapon that can melt/dissolve the gold.
If you are a Monk and have a nonhostile creature standing next to you, typing "n" to the "are you sure?" message when you use your circular kick results in the kick going off anyway. Bad, right? Not when you notice that it takes zero turns to execute and only hits hostiles.
That One Boss: The bosses of the two examples of That One Level below. The Ancient Chaos Wyrm and Nuurag-Vaarn are both powerful spellcasters, Nuurag-Vaarn even more so since he casts both Darkness and Death Ray.
That One Level: The Tower of Eternal Flames and the Mana Temple. The former deals constant fire damage to the player and burns his items, and the latter cripples spellcasters especially, as the magedoom eyes will drain their magic points. Both contain heaps of tough monsters, Chaos altars when stepped on will have the PC instantly sacrificed and two of the nastiest mandatory bosses in the game. Both places are usually regarded as the big turning points that put the power and skill of both the player and his character to the test - the tower especially, because beating it is required for reaching the latter half of the game and players are usually only about level 20 when they run out of other options.
That One Sidequest: Most of them. Even trying to save the little girl's puppy has a time limit and puts the PC through a dungeon with a guaranteed giant ant hill on the second level. Giant ants have sturdy natural armor and they are mean. (Additional fun awaits closer to the bottom of the dungeon.)