While exploring the caves between Yuri's island and the first island, along a path either just after or just before the room where you first see eggs (I keep going in circles and can't find the way out, so I don't even know any more), I spotted an area far below me with a faint light and a cross of that yellow-orange dot (what is that stuff? Mold? Paint?) on the ground.

Thinking it was the way out, I carefully hopped down a neatly arranged set of hexagonal stones. I found a +25 health buff sat on the cross of yellow-orange mold-paint. And nothing else.

And I can't get out. I can't jump back up the rocks I came down. I've tried. <i>a lot</i>. It is impossible, or possibly only through incredible luck. I've also tried to shoot the teleporter up there (because luckily I actually had the disk with me). It won't go up. It bounces off rocks, or it just doesn't go high enough.

This cave is impossible to exit, yet it was obviously intended for the player to be here or there wouldn't be a buff. The only conclusion I can make is this area wasn't actually play-tested and no-one realised that it's impossible to jump out.