* DemonicSpiders: In a game where enemies at your level can off you within 1-5 hits, a good number of them stand out.
** Red Longarms in the 2D version. Going melee with them is [[TooDumbToLive 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 OneHitKill 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 Reaper is this to pure Melee players who first visit the yellow zone. It has a BFS [[BladeBelowTheShoulder Below the Shoulder]] that deals ''very'' heavy damage in a [[InstantDeathRadius pretty big radius]], with only a blind spot behind it.
** The new Spider zombie is a ''literal'' example. They have good health and fast enrage speed, but worst of all is that as soon as they are aggro'd they go into their enraged state.
** Most variants of Zombie Dogs other than the basic one. Take the zombie dogs mentioned below, give them ''insane'' speed and damage for the "recommended" level, plus a good amount of health, and [[LightningBruiser you've got a deadly foe on your hands]]. The Blood Dog, found as early as the green (second) zone, hits as hard as the Reaper and can kill armored characters within 2 hits, moves ''extremely quickly'', and can take more damage than the Burned Zombies. Take the Blood Dog, double its already high damage and health to make it even more painful than the Tendril, give it ''even more'' speed, and you get the nightmare that's the Flesh Hound. Take the Flesh Hound, multiply its health and attack by 2.5x, and you get the Hell Hound which is thankfully only found in the Wasteland. Worst of all, since these ''aren't'' the uncommon mutant zombies, they appear ''much'' more frequently than most of the other DemonicSpiders listed above.
* DifficultySpike: In ''Winter Assault'' each campaign is less then half the length of the original (5 levels each in ''Winter Assault'', 11 in the original), but the later levels are equally difficult, i.e, difficulty shoots up very quickly.
** The fourth level of both ''WA'' campaigns deserves a mention, as in the Order version you are under almost constant attack by infinitely spawning enemies and in the Disorder version it's a race against time to stop ''both'' enemies completing their completely independant victory objective causing you to lose automatically.
* FridgeBrilliance: Why are Spiders so resistant to knockback? Their multiple arms give them a great bit of stability.
* GoddamnBats:
** Zombie dogs in 3D have very low health, but are very fast for an early game enemy, and can be a pain for melee players as their attack comes out faster. It also doesn't help that they have HitboxDissonance, causing many attacks to miss. Later equivalents are DemonicSpiders.
** Goddamn birds will wait until the very moment you open the chat prompt to descend en masse like something out of a Wagner opera. They disappeared in the 3D update... only to reappear in full force in the sequel.
** The sequel introduces parasites, which are small (and thus trickier to hit and detect in the dark) and appear in groups. Worse still, they start out in "Enraged" mode from the get-go unless the player is sneaking. Fortunately they deal low damage.
* ScrappyWeapon: 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.
** RescuedFromTheScrappyHeap: 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.
* TearJerker: The protagonist of the "Outbreak" games [[DrivenToSuicide 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 [[MercyKill shoots his wife]] before [[AteHisGun using it on himself]].