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.
Nightmare Fuel - People who have solitude issues or claustrophobia should probably think a bit before playing this game.
When you play Submachine Network Exploration Experience, the coordinates 666 land you in Hell. Visually it's a small room, dimly lit with red light, with a ladder that you cannot climb to the top of because it leads into absolute darkness... and the audio is loud, intense distortion noises that sound like demons growling and shrieking. Can also be Nothing Is Scarier, given that all the scary stuff in the room is only suggested or ambient.
Paranoia Fuel - Loads of it if you get the sense that you are being watched (like the hovering security cameras in Submachine Network Exploration Experience).
Sometimes in later games, you'll be able to visit places you've been to in earlier games and find that things have changed. Some of these changes are because of your actions when you were there before, but then there are... other changes, made by someone or something else, that are not explained. Like smashed lights, broken ladders, added Karma Portals, and Location 747's rubble cleared away to reveal a second corridor that seems to go on forever. The likely real-life reasons for these changes (practical and/or Rule of Cool) may or may not make this any less creepy.
Player Punch - In Sub 9 Murtaugh, the guy who has been directing you, manipulating you, speaking with you, and who had been your only likely chance for a human connection in the Subnet, is found dead. And not just dead. Mummified. Who knows how long he has been dead? You're alone and always have been.
And the same case for Elizabeth a little later on...