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: 3D Dot Game Heroes
Demonic Spiders: Knights. Only vulnerable from behind and deal lots of damage. One optional region (which is where Spelunker can be found) is chock-full of them in From Mode.
Ear Worm: Aaargh, the character customization music!
Its even harder in the Japanese version, where not only do you have to collect every Key Item, but you also have to beat both Dash Circuit Records, beat Blockout with no Misses, finish all 5 Tower Defense levels perfectly, and complete every single event in the game, most of which don't reward you, disappear after beating a dungeon, and have multiple steps that require you to revisit the person later on (Such as the Armored Core guy, if you forget to visit him between every dungeon you miss out on getting the Armored Core Key Item.)