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.
Fridge Brilliance: At first, the treasures you have to bring back at the beginning seem fairly random: a jeweled skull from a pirate cache, an acting award, and a cursed guitar. Then, much later, you find the corpses of the previous party of three to enter: a pirate, a clown, and a robot. Those were the things they desired most.
I was under the impression that the trinkets belonged to those who completed the Cave and made the wrong choice, since they're later replaced by your own characters who made the bad choices. So that would mean that, unlike the Pirate, the Robot, and the Clown, the seekers of the Guitar, the Skull, and the Award actually made it through to the end of the Cave.
Unless the Pirate, Clown, and Robot did make it to the end, and the Cave dumped their bodies there.
I assumed they were Double Fine or LucasArts references: A jeweled skull for Guybrush, a cursed guitar for Eddie Riggs, and Gloria Von Gouton's acting award from Psychonauts.
Fridge Horror: During the time Travel section, if you take someone to one of the earlier timelines, then walk around in a timeline later than that, you'll see a skeleton in the same spot you left them. Even moreso, this is required to solve one of the puzzles involving a boulder by holding it in place. The obvious implication is that your companion never left the spot they were standing and died there, even to the point of holding a rock in place. However, it does create a bit of fridge logic because despite the fact that up to a million years have passed and a history museum was built over the spot where they died, no one disturbed the corpse and their remains are left lying among the exhibits. Of course, this has plenty to do with the weird and timey-wimey nature of the cave.
Part of what makes the Twins so creepy is that when you are not controlling them, they are always staring directly at the fourth wall. And all they want is their freedom to go outside and play. Which means that you're in their way...
That One Achievement: Who Wants To Live Forever involves getting through a run without dying. Unfortunately, this is incredibly difficult to do, as there's plenty of things that can kill you in the game, such as falling to your death, mishandled dynamite, and being squished by floating platforms. Mistiming a single jump can force you to restart the entire game, even if you were near the end. You can't Save Scum either, as that runs the risk of deleting your save file, and it disqualifies you from getting it anyways. It doesn't help that the game has a tendency to glitch out at times, causing a completely random death to happen.