Brutal Bonus Level: Only available if you get all the gems and also introduces the final eversion. In earlier versions, it starts warping randomly between them all. Version 1.7 replaces it with a maze-like level.
Bubbly Clouds: After your first eversion, those clouds are nifty platforms!
Central Theme: The author based this game upon an idea from HP Lovecraft's Commonplace Book: "sounds - possibly musical - heard in the night from other worlds or realms of being."
Checkpoint Starvation: In world 6, unlike most of the others, the check point is between two-thirds and three-fourths of the way through the level, and it can be a pain to reach. It's right before the hardest part of the level, which helps, but you're going to be seeing the beginning of that level a LOT.
Completely Missing the Point: In the Crispy Gamer review of this game, the reviewer says about the game's recommendation to play alone at night "There certainly were not any special "alone at night" effects, nor any creative use of the system clock."
Content Warnings: The above quote, displayed when you start up the game. You're thinking that since this is a cutesy platformer at first, that it's just a joke. It's not.
Crapsaccharine World: The entire game world is falling into decay and ruin, occasionally getting eaten by giant walls of nothing before turning malevolent and irredeemably evil. The bad ending is when you get eaten by the princess, who's already been turned into a monster; the good ending is where you turn into one of the nightmarish creatures as well... but at least you get the girl!
Another interpretation is that you are a monster all along, and that you're working your way back from a horrible world to one that, for you, is good and right.
Given how evert points operate and the ending, a more natural interpretation may be that the world is a single entity with multiple faces, and possibly that World 8 is the true face. This would make World 1 a fake, or a Crapsaccharine World.
Cyclops: The Ghulibas' eyes start to cross in X-3 and are fused into a single eye from X-4 to X-8.
Dark Reprise: Does the music for X-6 seem familiar? It's an creepy, distorted version of X-2's theme.
Dark World: The entire point of the game is to progress between various stages of Dark Worlds.
Dramatic Disappearing Display: Your score goes away in X-7, then everything but the gem counter in 7-8, and the whole thing in world 8. In time attack mode, however, the timer is always displayed during gameplay.
Dual World Gameplay: Practically all of the gameplay is this to considerable success. There are 5 to 8 extra layers depending on version, but per level, you'll only be using 2 or 3 of them. At different layers, different objects may be obstacles or even enemies. In the lower levels, clouds can be platforms. In the lowest levels bushes are obstacles, go higher and they are simply dead, with flowers as thorny, spikey death traps, just before that they are just dead and background art. Each of these has an effect on the flow and timing of the player's jumps adjusted for the different levels. Enemies are deadly to different degrees with different one optimised for different levels.
Dummied Out: In normal gameplay, you will never see the X-8 gems since you'd have every gem before you can even see X-8. At least, before 1.7.3 introduced Time Attack Mode, which have the gems serving to reduce time. But prior to that they do exist, probably so going to X-8 with gems around wouldn't kill the game in earlier versions.
In addition, hacking the game reveals levels X-9 and X-0. 9 appears to be a Minus World (all the monsters look like you, and the level properties are whatever they were in the previous layer you were in). 0 was the world used in the map editor, with white tiles. All shrubs are replaced with the word and question mark "Goal?", which triggered the "bad" ending.
Earn Your Happy Ending: The first ending by completing world 7 is a bad ending. To get the better one, collect all gems and complete the bonus stage.
Easter Egg: In the newest version (as well as the HD), you get a secret ending if you end Level 7 on X-4 with all the gems. There's also a bonus screen for getting 100% completion.
Eldritch Location: As you continue everting, the world becomes more and more like this, peaking in X-8.
Endless Corridor: World 8 in version 1.7.3 onwards. If you don't find the evert point within two sections, then then you'll have to go through them again until you end the section after everting to lower dimension.
And almost faceless those stone enemies in world X-7.
Fake Difficulty: The demonic hands that emerge from World 6's Advancing Wall of Doom are more or less fixed in when they will emerge, but not where, meaning even the most skilled player can get killed by a hand that traps them between itself and another enemy or a hole, or even the wall, and there's nothing they can do about it.
Fission Mailed: In X-6 to X-8, dying will sometimes replace the "READY!" screen with a fake Game Over.
Ghibli Hills: Look at the green grass and blue skies shown in the page image! Isn't it a beautiful place? The HD version also puts nice hills in the background! Oh my god, what just happened to this place...?
Goth Spirals: The bright purple flowers from World 1 gradually wilt into gray stalks that become spirals in World 5. As if that weren't enough, they acquire Spikes of Doom from World 6 onward. In HD version, more can be seen in the background during later worlds. And let's not forget what happens to the face blocks in World 7...
Guide Dang It: Most players who play the new version of the World 8, take the hardest path to get from world X-6 to lower instead of everting from X-6 to X-5 from the easier section.
Getting those last five gems in level 6 (or even getting to the end of the level) is frustratingly difficult, to say the least until you find the hidden eversion point in the maze.
Level 7 and those last few gems... and just when you think you've got the blocks destroyed and the gems popped out, if you should ever die in between grabbing the gems, YOU HAVE TO DO IT ALL OVER AGAIN. AARGH. Cue the head-smashing.
Partly averted as most Guide Dang It moments are shortly after automatic saves and you have infinite lives to try everything. In the first example given, you can get trapped within the easier section, forcing you to search for the evert point within a small area.
Heartbeat Soundtrack: X-7. X-8 features accelerating heartbeats. X-5 also has a very slight heartbeat.
Interface Screw: The later parts of worlds X-6 and X-7 are plagued by creepy glitches, messages, and other oddities in your UI. For example, your score starts turning into gibberish — then vanishes outright; instead of "READY!" it starts saying things like "STOP," "GIVE UP," a fake Game Over, etc etc. In the endings, the game closes itself.
The HD version has an extra special ending where Zee Tee and Nehema are warped to X-4 and frozen solid.
Musical Spoiler: Before you evert, you'll hear what you're everting into. Every time you evert to a higher layer, the music gets less and less cheerful. First it mellows out. Then it becomes dramatic. Then it becomes weird. Then it becomes oppressive. Then you hear the mellow music as if played on a broken record player. Then all you hear is a beating heart. And after that, cue the Psycho Strings...
Noob Bridge: Many people get stuck in world 2 and fully learn how the mechanics work only when they reach world 3-4, especially in non-HD version. Eversion points are easy to miss if you don't know what to look for, and people often ignore the eversion point in 1-1 when they walk over it.
Not Quite Back To Normal: The second version of the last level has Zee Tee travelling from Layer 8 to Layer 1. When you reach the end of the level, a red hand comes out of the floor. Although unlikely to hit you, it shows that the world has been distorted beyond repair.
Scenery Gorn: In the High Definition version, the Ghibli Hills in the background grow decayed. X-2 makes everything look artificial and X-3 makes it look like it's falling apart. X-4 just shows a flat, completely stagnant ocean. The hills grow CLAWED HANDS and MOUTHS in X-6 and X-7. And then we have LAYERS and LAYERS of TENTACLES in X-8.
Schmuck Bait: To those who didn't have this game spoiled for them: admit it, you were wondering why the cutesy-looking platformer game was marked as "not for children or those of a nervous disposition". The fact that it starts with a quote from Lovecraft should have clued you in.
Take That: The 'Metaphor for the Atomic Bomb' achievement takes a jab at another Mario-esque platformer with an added mechanic. The tone is evident from the icon for the achievement: Zee Tee and Nehema making the 'awesome' face.
Unwinnable by Mistake: Some level designs can lead to the player getting stuck in one place without being able to get out or die, and the level must be restarted. Some instances of this were corrected in later versions, but it's still possible to purposely enter a unwinnable state in some locations. Fortunately, you can quit the level and restart without losing any of the gems you've collected during that level!
Tomato in the Mirror: The other interpretation of the "good" ending. The distinction might be academic...
Trial-and-Error Gameplay: Plenty of it, but getting a hundred percent completion on world 7 truly goes above and beyond—it's likely to take at least as long as the entire rest of the game put together to finally get those last six gems.