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: Red Longarms in the 2D version. Going melee with them is asking for it. In the 3D version, the Bone and the Tendril. He is fast and can take a decent part of damage (including several grenades and a few clips of assault rifle) before dying and is pretty large. She can One-Hit Kill a player with a low endurance even with an armor in perfect state and full health and has a decent range. They're ALWAYS escorted by a crapload of regular zombies who run at you like there's no tomorrow. And the deeper you go in the city, the more often they come to say hi...
The new Spider zombie is a literal example.
The red zombie dogs. Insane speed, massive damage, and automatic aggro if they catch sight of you.
Goddamn Bats: Goddamn birds will wait until the very moment you open the chat prompt to descend en masse like something out of a Wagner opera.
Tier-Induced Scrappy: Shotguns. Despite requiring a substantial amount of strength to wield and being, well, SHOTGUNS, they're indisputably the worst weapons in the game, being surpassed by equivalent handguns in almost every department.
Rescued from the Scrappy Heap: Shotguns again. In the new 3D update, shotguns are now no longer useless, as pistol and rifle damage has been nerfed, and knockback and hitstun have been nerfed for every weapon EXCEPT shotguns.
The roleplay classes are seen as this. They have no useful skill or bonus except gaining more XP
Tear Jerker: The protagonist of the "Outbreak" games hangs himself in an old barn at the end of the 1st game if he leaves for the countryside without saving his beloved wife. Thankfully not canon with the 2nd game.
The bad ending in the second game isn't much better. If the protagonist returns to the farm without getting the antibiotics needed to cure the cholera that has been infecting many people in the farm including his wife, he steals a gun from the armory and shoots his wife before using it on himself.