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.
Demonic Spiders: Lots of examples, particularly enemies that are immune to magic, can One-Hit Kill you, cast Sleep, Healmore/Midheal, and/or Hurtmore/Firebane/Sizzle (Wizards), inflict poison or other Standard Status Effects, and have high defense or speed stats (which makes them dodge and block you a lot, eg Demon Knights).
Hell Is That Noise: Put a piece of cursed equipment on, or have a save file erased in one of the NES games for failing to hold down the reset button while turning the power off, and you get treated to a creepy low-volume Scare Chord.