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.
The fact that quite a bit of the back story can only be found at the forums certainly doesn't help.
Demonic Spiders: Sneaks (A type of snake) are probably the most common, but they're getting edged out of their position as "King of all monsters that make players tear their hair out." The most obnoxious thing about them is that they're very, very difficult to hit with anything except a ranged attack unless your character has high dexterity. However, some more-recently-introduced monsters are just as hard to hit or worse, and they don't have the ranged weapon weakness. And since status effects were introduced fairly late, monsters that use them are really, really obnoxious.
Good Bad Bug: During the "Divide by Zero" war, the monster pool was abnormally strong, leading to waves of monsters that were 30 levels above the character's level. This also meant farming was made easier, but slowed war progress.