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.
Brilliant!. Earning Five Stars in every level, requires dying less than three times (five in some levels) and picking roughly 85~90% of the gems. Keep in mind that after 1-5 the rest of the entire game is made of Marathon Levels.
Harder than Quartz, Harder than Obsidian and Harder than Diamond. Awarded for beating worlds, not levels, one, two and three, respectively in Über Hardcore. Have in mind that dying once in Über Hardcore means Game Over and you'll get thrown to level ONE.
That One Attack: The final boss' last phase. Gurglewocky will raise lava up the the middle of the screen while dropping several fireballs. In Normal difficult upwards you won't have a single stable point to stand on, as the platforms in the arena are of the crumbling type, and sticking to the wall is generally a very bad idea during this attack.
That One Boss: Octor Freud, his erratic succession of attacks becomes faster and more dangerous every time he is damaged and his homing attack, which he uses his vulnerable point, can be difficult to dodge.
That One Level: 1-6, Amethyst Mines, might come as a shock to players as it is the first lengthy level, it is also when the game's difficulty raises considerable requiring precise jump timing and has an absurd amount of spikes with moving platforms gimmicks never presented before. To top the level is lair of the first boss, Hansel and Gretel.
3-7, Boiling Springs, gets pretty nasty near the end where you have to float down a vertical corridor lined with fireball shooters and a part where you have to get from extremely narrow platform to extremely narrow platform above a moat of acid while deathtraps are orbitting said platforms. One false step and you can do the whole thing over again.