* DemonicSpiders: 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.
* GoddamnedBats: Swarm Drones and Buzzers. The first is your regular CannonFodder 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 [[InterfaceScrew screen shake]] and your character stumble.
* MostAnnoyingSound: "Medic! Medic! Medic!"
* TheyChangedItNowItSucks: The Tesla Cannon received a nerf, reducing its damage on enemies and making it drain ammo should the player keep firing without any targets near. Before the nerf, [[ComplacentGamingSyndrome almost everyone picked the Telsa Cannon because it killed lesser aliens quickly and they could hold the trigger without wasting ammo if no aliens were near]]. The gun still has its uses, but now everyone complains that it's garbage. It still has its great ChainLightning stun effect, though.
** 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 its 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.
* ThatOneLevel: The Syn Tek Residence. The [[DemonicSpiders Swarm Parasites]] are partly to blame. Also to blame are the sprawling, nonlinear level design that will make sure at least one of you gets lost, and the omnipresent [[GoddamnedBats Buzzers]]. [[KillItWithFire Bring]] [[FireBreathingWeapon flamethrowers.]] [[ThereIsNoKillLikeOverkill Give every Marine a flamethrower.]]
** The developers seemed to have realized this as of ''Reactive Drop'' where lower level players attempting to play through the Syn Tek Residence level will now be allowed access to the flamethrower, even if they don't have it normally unlocked.