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.
Ambiguous Gender: The short-haired Kaibutsu models have been interpreted as both male and female, due to the ambiguity of their sprites. However, the twintail model is almost always interpreted as female.
Demonic Spiders: The Kaibutsu, which are pale white-haired figures resembling schoolchildren with bleeding faces. Usually they simply wander around, minding their own business, but then if you smack them with the steel pipe, they become enraged and chase you. Touching them teleports you to inescapable areas. Some of them don't need to be smacked to be enraged, since they already are. Luckily, you can avoid them by using the ghost effect's invisibility, but touching them still teleports you away.
Crack Ship: Sabitsuki is sometimes shipped with the Kaibutsu.
Ear Worm: Many of the songs, scary or not, but especially Shinsoku Neko's music.
Ensemble Darkhorse: Quite a few characters, such as Oreko, Smile and Sister, and the Kaibutsu. Sabitsuki herself is very popular, not unlike Madotsuki and Urotsuki of the other games.
Goddamn Bats: The red demons. Though they only appear in one part of the game — specifically, the Womb Level near to where you get the Viscera effect — they nastily disable your effects upon contact, not to mention there are a lot of them in said area. In the endgame, though, they become even worse. They're just about everywhere at this point in the game, they slowly wear Rust down, move erratically, and have an annoying habit of speeding up when they have a clear path to Rust. On the other hand, they make certain spots that normally had Kaibutsu easier as they replace a lot of them, meaning Rust can just tank the damage and move on. Except that if you touch too many, you explode in a bloody mess!
Les Yay: Some of Oreko's interactions with Sabitsuki, as well as the existence of the Friendship Meter, are interpreted as this.
Nausea Fuel: All over the place. The deeper levels of the body area, the Viscera effect, the Dying Girls and Bloody Room events are a few of the most prominent examples.
The Orange Menu is obtained from a ghostly Oreko in an area where melancholic music plays, implying that Oreko died at some point, but still wants to help in anyway she can.
In the children's hospital, you can find a younger version of Sabitsuki. She's bedridden and attached to an IV. If you level up the Corruption Meter, her condition deteriorates and more wounds appear on her body until she looks inhuman.