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.
YMMV: Alien Swarm
Demonic Spiders: Swarm Parasites. When they're not on someone, they're easy to kill. When they are, it's usually Game Over for the guy if there are no medics in the team. An unlockable armor helps against them by shocking them if they try to attack you.
Even with the medic, they are damn hard to cure, especially with more aliens bugging you. And some players tend to run around like headless chickens either way. They chew up health like nothing else, taking about an entire health beacon to cure (and another to fix the damage). And for extra fun, when they kill someone, about eight more burst from the corpse, often leading to a chain reaction that kills the entire team.
With the medigun, it takes two players out of commission - the infectee, who's running around like a headless chicken, and the medic, who's trying to heal him up before it's far too late. Needless to say, if the player carries on sustaining damage, the medigun won't help...
If another player uses the Tesla on the infectee, the rate of infection is slowed so the medic has a much greater chance of saving the player. Course, then you have three players tending to one parasite.
Goddamned Bats: Swarm Drones and Buzzers. The first is your regular Cannon Fodder enemy, the latter is a little bat alien thing that swoops down and hit you in the face. It doesn't do much damage, but it makes your screen shake and your character stumble.
Temporarily averted by a glitch that caused it to receive 25 bonus damage for each point a marine had in the damage bonus stat. Given that it's pre-nerf damage was only 20 (and its base damage at the time of the glitch was 1), this made it the most deadly weapon in the game, until it was fixed. Amusingly enough, the gun that shot electricity was actually better at blasting through rock that the mining laser.