1- The oxygen tank :
The oxygen tank explodes when it falls from a great distance.




2- The Witch and the certain walls :
The Witch is able to hit players through certain walls, so be careful and stay on a safe distance.




3- Gnome Chompski :
In the campaign "Dark Canrival" you can find Gnome Chompski and eventually rescue him for an achievement. That Gnome is hiding on the Dark Carnival Poster.




4- Jimmy Gibbs Jr. :
It is possible to see the famous car driver Jimmy Gibbs Jr. as a zombie. He is considered an idol in the eyes of Ellis and Coach. Jimmy Gibbs Jr. is dressed in his racing suit and only spawns in The Atrium, with a 5% chance of spawning, so he is extremely rare to see in most games also he has 1000 hp. He is Immune to fire, oil blinding, immune to lure of Pipe bombs and Bile bombs.




5- Reviving from fall :
You can't revive survivors who died from fall after hanging on a ledge, don't waste your time to try.




6- Shooting through the walls :
When you shoot through the wall you won't hurt your teammates but you can hurt the infected (even the special ones).




7- Lambda :
This is reference to the game "Half-Life" made by Valve. You can find this easter egg on the campaign "Swamp Fever".



8- The Tank :
The Tank can hit you even behind him if you get too close to his back.




9- Tank's speed :
The Tank moves slower than the survivor by ~4,5%. Survivors have a base speed of 220 velocity, the Tank has 210 and his speed drops when he is being fired at. Fire doesn't affect Tank's speed in versus mode. In Campaign mode the bot Tank's speed increases.




10- Tank's rocks :
The Tank's rocks can be broken by throwing something at them, like gas cans, pipe bombs and etc.




11- Portal in Left 4 Dead 2 ? :
With any jukebox that you find, it is possible to hear the song "Still Alive" from the game Portal which is made by Valve too. If you want to play it you'll have to keep changing the songs until you find it.




12- Movies and Games references :
In the campaign The Passing, in the "Red Flight" Bar cafe you can find a room with a pool table. On the walls you can see references from famous games and zombie movies such as Dawn of the Dead, Dead Rising, Shaun of the dead, 28 days later, American Dad and the game Plants VS Zombies.




13- The Bots will stop healing you :
Bots will stop healing you if you take out meds.




14- Bots receive less damage :
Bots receive less damage from the Spitter's goo than the players.





15- Genius Bots :
Bots will not trigger any car alarms when shooting at them but they CAN trigger any car alarms when walking or jumping on them.