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: The literal spiders (aka Zigmos) can make some of the later levels unwinnable if you lose your weapon. There's also those pesky frogs/Kellos. The literal Goddamned Bats can be particularly demonic when platform jumping. And the jumping swordfish come at you almost-nonstop in the later ocean levels.
Goddamned Rocks. Bounce, bounce, bounce, die, throw your controller.
And there are also bassers which are bats which often qualify too.
Guide Dang It: The success of button-mashing minigames in Adventure Island IV depends on how much food you have in inventory before entering minigame.
That One Level: Although the whole world 8 in the first game is very hard, the world 8-3 is especially noteworthy due to a segment which requires pixel-perfect jumps and firing.
