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 Retardant: Not the mod itself, which managed to make Houndeyes terrifying. In SvenCoop, it's a popular server, but obviously, having a buddy with you mowing down hordes of twitchzombies lessens the impact a bit. The official server works to make this less effective (players are often split up and cannot speak to each other unless near each other, they bleed out when on low health, and ammo and weapons are as scarce as ever), but the snark of your fellow teammates and your laughs at their anguished screams kinda dumb it down a bit.
Originaly, there was supposed to be zombies with big white eyes. They did not look frightening as much as weird, and were cut.
Real Song Theme Tune: The game uses various tracks of Aphex Twin, the majority of which are from one of his most acclaimed albums, "Selected Ambient Works Volume II". In fact, in the first 10 minutes of the game you can hear 4 tracks of the same album in a row - Stone In Focus (Main Menu), Hankie (Intro), Cliffs (before blackout) and Tree (after blackout).
That One Level: The Hospital after the blackout. People tend to quit playing the game when they get lost too much.