Remain idle at the game selection screen. After a few minutes, a demo video of one of the games will begin. After the video ends, wait for the next video to begin, which will be a Left 4 Dead trailer.
Accomplish the indicated achievement to get the corresponding number of Gamerscore points:
Additionally, there are eleven secret achievements:
While playing the game, press Left(4), LB, Right(4), RB to unlock all levels.
While playing the game, press LB, Up, RB, Up, LB(2), Up, RB(2), Up.
While playing the game, press Up(2), Down(2), Left, Right, Left, Right, B, A to spawn a medpak that will restore 25 points of health.
While playing the game, press Y, B, A, X, RB, Y, X, A, B, RB to spawn an ammunition cartridge for your current weapon.
While playing the game, press Up(3), LB, Up(3), RB, Up(2).
While playing the game, press Down(3), LB, Down(3), RB, Down(2).
While playing the game, press LB(2), RB(2), LB, RB, LB, RB, Left, Right.
While playing the game, press Y, Up, Y, Right, Y, Down, Y, Left, Y, Up.
While playing the game, press LB, Down, RB, Down, LB(2), Down, RB(2), Down.
Successfully complete the game to alternate the title screen.
When you need to run a long way and have some objects with you (for example, a non-explosive barrel), if you have the Gravity Gun, use it as cover, and run (while facing the enemy) to the other side. Then if possible, shoot the barrel at the enemy. Note: Make sure there is only one enemy when doing this. Also, make sure the object is indestructible, and most importantly, look at the enemy and see if you can kill it with one shot.
While playing the game, press Left(4), LB, Right(4), RB to unlock all levels.
While playing the game, press LB, Up, RB, Up, LB(2), Up, RB(2), Up.
While playing the game, press Up(2), Down(2), Left, Right, Left, Right, B, A to spawn a medpak that will restore 25 points of health.
While playing the game, press Y, B, A, X, RB, Y, X, A, B, RB to spawn an ammunition cartridge for your current weapon.
While playing the game, press Up(3), LB, Up(3), RB, Up(2).
While playing the game, press Down(3), LB, Down(3), RB, Down(2).
While playing the game, press Left(4), LB, Right(4), RB to unlock all levels.
While playing the game, press LB, Up, RB, Up, LB(2), Up, RB(2), Up.
While playing the game, press Up(2), Down(2), Left, Right, Left, Right, B, A to spawn a medpak that will restore 25 points of health.
While playing the game, press Y, B, A, X, RB, Y, X, A, B, RB to spawn an ammunition cartridge for your current weapon.
While playing the game, press Up(3), LB, Up(3), RB, Up(2).
While playing the game, press Down(3), LB, Down(3), RB, Down(2).
Set the system date to October 31. After you are ambushed by Hunters and Combine at a big house, drive along the road. Near where the supply cache is found, you will see an old truck. Go up to it, and there will be lots of pumpkins lying on the ground.
When you enter the White Forest base, you meet a Vortigaunt in a lab coat named Uriah. When you first enter the room where you meet Uriah, there is a gas tank against the back wall. Behind that tank is a small room with a computer that has a sequence of numbers on it. This is a reference to the TV show Lost that Gabe Newell added due to the fact that he and the creator of Lost were close friends.
While playing the game, press Left(4), LB, Right(4), RB to unlock all levels.
While playing the game, press Down, B, A, B, Y, Down, B, A, B, Y. Note: Enabling this code will prevent achievements from being earned.
While playing the game, press Up, Y(2), X(2), A(2), B(2), Up. Note: Enabling this code will prevent achievements from being earned.
While playing the game, press Up, Y(2), B(2), A(2), X(2), Up.
While playing the game, press Up, Left, Down, Right, Up, Left, Down, Right, Y(2).
While playing the game, press Up, Left, Down, Right, Up, Left, Down, Right, X(2).
While playing the game, press Up, Left, Down, Right, Up, Left, Down, Right, A(2).
While playing the game, press Up, Left, Down, Right, Up, Left, Down, Right, B(2).
While playing the game, press Y, A, B, A, B, Y(2), A, Left, Right. Note: Enabling this code will prevent achievements from being earned.
While playing the game, press X, B, LB, RB, Left, Right, LB, RB, LT, RT. Note: Enabling this code will prevent achievements from being earned.
Successfully complete the game to unlock the cake title screen.
The graffiti in the game are clues for the ApertureScience.com website. Go to the site, and enter "LOGIN" as a command. Enter "CJOHNSON" as a username and "TIER3" as a password. After logging in, type "HELP" for a list of commands. In addition to logging in as "CJOHNSON" (Chell), you can login as anyone desired by typing a username longer than two characters and using "PORTAL" as a password.
While logged into the ApertureScience.com website, type "THECAKEISALIE" to access a hidden message left by some employees.
At the end of the advanced map for Chamber 18, you will have the same raised platforms as you did in the normal one except now after you leave the lower one, it falls into the toxic water. This forces you to be more precise with your portal jumps. However, you can get past this easily. Do the platform jumping as usual, but if you miss, just land on the platform you came out of. You should be able to fire an opposite color portal at the wall where you entered this area, allowing you to go back to the start. All the platforms rise again, and the portal is still where you left it. You can now fall into the pit again and be able to portal jump to the next ledge. This take longer but does not require precise timing.
On the level where you get the companion cube after you run past the energy balls and are in the room with the three elevators, there are two doors leading to a catcher and a platform that is pushed out of the wall. Walk up to the platform that is pushed out. Crouch behind it, and walk through the opening. On the left hand side are a group of pictures of people with boxes for heads and hearts around them.
Play the level where you face off with the laser guided robots. Once you get past the first few robots, you must start dropping the boxes on the robots heads so they fall over. In the little area where you find the first two boxes, crouch and walk behind where the boxes were. You will find a little room with odd music playing and graffiti on the walls that says things such as "The cake is a liar". There also seems to be an Xbox 360 with a pot inside of it. The Xbox 360 is overheating and boiling the water.
As an engineer, do not wait for your building. Just whack it with the wrench so that it builds faster.
Play a game with just you and a friend. Have your friend let you repeatedly kill him or her to easily get achievements.