Easter Eggs
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Source SDK
1337
In the July 2008 SDK release, the number of valid AI activities defined by Valve in ai_activity.h is exactly 1337.
Hammer Editor
Go To Coordinates
If you attempt to go to coordinates 200 -4096 1154 using the Go To Coordinates dialog in 
Hammer, the following dialog appears:
This is in reference to the example coordinates given when you first open the dialog.
Hammer Time!
In Worldcraft (later renamed to Hammer) 3.40 from Build 2513 (including the 2003 
Half-Life 2 leak), when you run it for the first time, a MIDI of "U can't touch this" will play, and the text that usually says "For Help, press F1", instead shows any of the following strings: "You can't touch this", "Stop, Hammer time!" and "Hammer time!". This lasts for about 3-4 seconds before returning to normal. Once activated, this Easter Egg will no longer activate again, only reactivated by changing a value in the Registry, specifically the value "Hammer time" under HKEY_CURRENT_USER\SOFTWARE\Valve\Hammer\General. This Easter Egg doesn't activate on modern versions of Hammer, as a search through the .exe in a hex editor shows that the string "Hammer time" isn't found in the file.
Question: When was the Easter Egg removed?Day of Defeat: Source
WTF
In 
Day of Defeat: Source, the voice command voice_wtf makes the player say Whiskey Tango Foxtrot!.
Half-Life: Blue Shift
- Chapter 2: Insecurity
- Get the gun and all the ammo at the shooting range (17/51 in total) and go to the locker room. Open the locker that says "Calhoun" - and you will find a box. Shoot it until it breaks, revealing a chumtoad (purple alien frog).
- In the locker room, there is an out-of-order toilet. Climb the seat of the nearby toilet, and jump to peek at the back of the closed door. You'll find a poster of a face.
- At the monitoring stations, if you watch the monitor showing the two scientists long enough, they'll meet up and do a little dance.
- Chapter 4: Captive Freight
- After taking the hidden elevator down to the old lab, climb atop the broken duct and look into the intact part to discover another face.
- Chapter 5: Focal Point
- While exploring Xen, you can find a large pool. If you dive in and swim all the way to the bottom (make sure to use God Mode), you'll find a wall with a crack in it. Break the wall, and you'll find the entrance to Chumtoad's Lair.
- While very difficult, you can enter Chumtoad's Lair without God Mode or Noclip. It requires a large amount of health and a couple of resurfaces for air, so make sure you're prepared before entering.
- While exploring Xen, you can find a large pool. If you dive in and swim all the way to the bottom (make sure to use God Mode), you'll find a wall with a crack in it. Break the wall, and you'll find the entrance to Chumtoad's Lair.
The above easter eggs above are demonstrated visually in these videos:
Captive freight face and Calhoun chumtoad
Locker room toilet face and dancing scientist
Half-Life: Opposing Force
Duke Nukem?
half-life\gearbox\sound\drill\dr2_add04.wav: "It's time to kick ass and chew bubble gum, and I'm all outta bubble gum."
This is a reference to the iconic line, first used in "They Live" from 1988, later used more famously in the 1991 game Duke Nukem.
Win the Game
The unused sound, half-life\gearbox\sound\misc\dsbossit.wav, after audio processing has been applied: "To win the game you must kill me, Randy Pitchford."
This is a reference to Doom II, whose final boss (Icon of Sin) played a reversed and pitched-down sample of John Romero speaking a similar sentence.
Audio processing used: GoldWave → Effects → (Reverse + Playback Rate → 16000 + Filter → Noise Reduction → Initial noise)
Haiku
Open the console and enter haiku. Text will appear onscreen of a haiku, put together using random lines. Every line can be found in the file opfor.dll.
Half-Life
Dave Riller
Beneath a set of stairs in "On a Rail", you'll find a crate. Break it, and destroy the vent cover that was underneath the crate. Crawl into it, eventually reaching a HECU outpost with some satchels, a dead marine and a headcrab. On a wall to your left, you will see the word "RILLER" made out of brushes. This is a reference to Dave Riller, a level designer for Valve. It is a bit hard to spot at first, as it has the same texture as the walls.
Half-Life 2
Water Hazard
In the last map of the chapter, you arrive in a wide and muddy area, bordered by the dam leading to Black Mesa East, containing a combine warehouse and an attack helicopter.
After defeating the helicopter and opening the gate, go through the warehouse and grab all health kits and suit charge (it will help immensely). Next, drive until you find a wall with two pipes; the right one is blocked but the left one is open. Climb into the tunnel using the airboat. Once you're ready, sprint through the toxic water (jumping reduces how long you're in it) until you find a split in the pipe. Take the turn, and continue sprinting until you find solid ground. Here, you find the singing vortiguant, who sings to the player and talks about the story. While you're listening, take a moment to absolutely slam the health kits and suit charge they give you in the cave. Once you're done, repeat the steps you did to enter the cave, but backwards.
LOST (TV Series)
Behind a large white boiler in White Forest, there is a monitor displaying the text ">: 4 8 15 16 23 42", as well as a combination of the White Forest and Dharma logos. This is a reference to the TV-series Lost.
The monitor
The combined White Forest/Dharma logo.
Team Fortress 2
Team Fortress Birthday
Activated on the birthday of the original Team Fortress (both August 24, when the Team Fortress was released as a Quake conversion, and December 22, when the game was released standalone on the QuakeWorld 2.0 engine), all players wear birthday hats, all gibs turn into birthday presents, and any time you hit someone with a weapon, balloons slowly float away out of them.
This can also be activated with a birthday cvar, creating the side effect of "birthday servers", servers who constantly have the birthday effects on.
A birthday package with the name "gib01"
A birthday hat
Portal
A funny wet floor sign, found inside the "portal\portal_pak_dir.vpk" archive in the "materials/models/props_facemovie/wet_floor_sign" directory, with the text "THIS IS SPARTA!" sprawled across it in tape. This is a reference to the 2007 film 300. Interestingly, no matching wet floor sign model exists elsewhere in the files for this texture to be put on.
Counter-Strike: Global Offensive
The Boiler
Question: Does this belong here?In the map "de_inferno", there is a room in Apartments that is referred to as "Boiler". On the games release, it was simply a closed door, however fans soon noticed it was accessible in 
Counter-Strike: Source. After an update to 
Counter-Strike: Global Offensive, the door was slightly open, revealing a boiler inside. Later, another update moved the boiler to the truck outside "Ruins" at B Site.