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.
Demonic Spiders: The Siberian brothers, those Gasmask Mooks wearing body armours. Not only they can take more damages than their regular counterparts, but they are usually equipped with VIVDA KSK, a kind of automatic rifle firing explosive slugs which can One-Hit Kill Anderson even in Easy mode! Oh, and they are often met in corridors or closed areas, which make them even more lethal. Their dangerousity is actually a (tiny) bit reduced compared to what you expect: first, their gasmasks is only cosmetical and doesn't protect from mustard gas and LSD grenades; second, a VIVDA KSK magazine only holds five rounds so then must reload often.
Goddamn Bats: Well-hidden snipers and rabid dogs with suicide bombs attached are particularly annoying.
Paranoia Fuel : The use of sound effects is often very atmospheric, especially in the levels taking place in a dilapidated abandoned town. There's also one specific Doom Doors sound that makes the Ghost Town scenery genuinely creepy.