In the Normal Test Chamber 17 there is a piece of the level that's out of bounds. It was only meant to be accessed in the Advance Test Chamber 17, which is unlocked after the game is completed. It contains a momentum test with a glass panel that has a hole in the middle. This room can be accessed with cheats by typing 'noclip' into the console. It is also possible to access the room without cheats by using the 'Portal edge Glitch' on PC or it can be accessed on console and pc using a glitch called 'Portal clipping'.