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: Crystal Caves
Goddamned Bats: Not the bats, oddly, but the birds. They fly up high in the air (meaning they are out of your shooting range most of the time) and they drop eggs on you if you're below them. And if you avoid the egg, it will stay on the ground for a few moments and remain harmful during this time. And sometimes the egg will hatch into another bird. Aaaargh.
That One Level: Crystal Caves 2 contains a level which is essentially a giant, empty room full of Invisible Blocks which have to be hit from below to appear. You have to make them all appear so that you can get up to the exit. But doing so will require you to climb up all the blocks you've revealed so far, jump as far as possible so that you will (hopefully) hit the next block on your way, and then watch helplessly as your hero tumbles down back to the ground and has to climb up the blocks all over again. This level will take you a long time to complete.