Useful Notes! Anything that requires more than two sentences to explain is and belongs on the other L4D Wiki.note If we're going to use this as a tips page, we need to KEEP IT CLEAN AND BRIEF. Zero tolerance for discussion on the page, otherwise it's somewhere between a fan board and another Wiki: which would negate what we're trying to do with the page.
Items and Weapons
Healthpacks: They heal 80% of your injuries (cancelling all temporary health) and reset your incap count, rather than healing an exact amount of health. The more health you have, the less effective healthpacks are, and you can only heal up to 99 health before they stop being effective at all.
"International" version weapons: If you come across a weapon you haven't seen before in L4D2, it's because you're playing with someone that has the German version of the game or are playing a server that enables them.
(L4D2) A Bile-Bomb will spawn a horde if there aren't enough Infected nearby for it to attract
(L4D2) Using Adrenaline allows you to run at maximum speed, even when water or infected hitting you would ordinarily slow you down. (i.e. Hard Rain)
Hunter pounce damage: Not determined by how high above the survivors you reach, but rather how far the spot you jump from is to where you land.
Boomer vomit has a sort of cooldown effect. If a boomer explodes on someone that has been vomited on, the effects won't stack and will wear off when the vomit was timed to normally.
Teamwork is just as important for the infected team as it is for the survivors. If you don't attack with your teammates, even mediocre survivors will make all your attacks worthless.
All playable infected make noise when they spawn or attack. If someone killed you quickly after they spawned, they picked up on your location with surround sound and shot through a wall.
In L4D Campaign Mode, Tanks run faster on fire than survivors can run and have a scripted death after ~sixty seconds rather than take damage from the fire.
You can free Survivors from Hunters, Smokers and Jockeys with a melee/shove attack.
Chargers must be killed for their victim to be freed.
When a Smoker is pulling a Survivor towards them, you can free them by meleeing the Survivor or the Smoker, killing the Smoker or shooting the tongue itself.
When a Survivor is snared by a Hunter/Smoker/Jockey/Charger, bullets will pass straight through them.
(L4D) You can distract a Tank from his incapacitated target with a melee attack (provided the incapacitatee isn't shooting the Tank)
A Tank will always chase after a Survivor using a mounted machinegun.
You can destroy the rocks thrown by Tanks with gunfire.
Friendly fire adjusts depending on the difficulty (Easy is 0%, Normal (and Versus/Scavenge) is 10%, Advanced is 50%, and Expert is full damage).
Survivors always vocalize when they are pinned by special infected or have spotted them.
Other Useful Notes
A Boomer's vomit ALWAYS overrides a Bile Bomb's effects, but not that of a pipebomb.
Witches will chase after someone who disturbed her, but she will always go after someone else if they set her on fire.
If you're directly between a Witch and her target, she might just jump you.
Chainsaws and guns will attract idle common infected to you due to noise.
The German version of the game is censored for violence, and is more difficult as a result (you can't tell if a Hunter is on fire because the flames would be censored) so a number of weapons are ported from Counter-Strike. The Australian version of the game is also censored, but they don't get bonus weapons
Actor Existence Limbo: Bill gets this, and is later McLeaned since Bill's voice actor had such a busy schedule and had no time to provide new lines for the character. This causes Bill to be Killed Off for Real in The Passing DLC. Valve then reverses the trend by creating The Sacrifice DLC, a campaign that shows how Bill (or any survivor of your choice) meets their end. This time, Bill's voice actor returns and provides new lines for The Sacrifice.
Ascended Fanon: During the development of Left 4 Dead 2, Valve noticed that the community loved making things explode and wanted more like it, so they tossed in the Grenade Launcher just for that.
Beam Me Up, Scotty!: Although the line "WHOO! BUMPER CARS!" is often misattributed to Francis thanks to popular Garry's Mod videos, he never actually says that in either of the games. The voice clip used in those videos is actually that of Mattias Nilsson from Mercenaries.
Hey, It's That Voice!: Most of the NPC Survivors are played by guys who also did voices for Team Fortress 2. For the curious, the boat man is the Sniper, the church guy is the Scout, the infected helicopter pilot is the Spy and Pyro, and the pilot in Dead Air is the Demoman and Heavy.
Valve is infamously known for their Valve Time where they either set a date for a release for something and it gets delayed (sometimes several times) or they don't mention a release date at all until the product is good and ready to ship out. Left 4 Dead 2 is one of the few exceptions to the rule; the game was released one year after the original Left 4 Dead due to Valve wanting to prove that they can release on a fixed schedule. This action caused a Broken Base and it's best to leave it at that.
In The Passing, there were rumors of using a defib unit on Bill to bring him back to life. It was quickly proven falsenote his body does not have a red outline when you hold out the item and his body is propped up against the generator, which wouldn't play the defib animations correctly since all survivors that die always have a set pose for this, and a few patches later they added a unique message for people attempting it: "Bill doesn't need revival."
One myth that many believe to this day is healing each other with first aid kits instead of using them on yourself will make the director spawn more first aid kits due to using more "teamwork." There has been no concrete proof of this.
There was originally a Special Infected called the Screamer, which ran away while spawning enemy hordes. This was before the concept of Boomers attracting zombies existed, as they originally just did damage when they exploded.
There was also a zombie called the Leaker which planted itself like a landmine and exploded into fire when survivors got near. A half-working version of it with the Boomer's model can be played as via console commands.
Valve considered calling Left 4 Dead 2 "Back 4 More".