There are a couple of areas in the game where you can trap yourself in a test chamber with no way to get out. Once the developer figured out that the player could do this, they programed GLaDOS to open an escape route which earned an achievement called "Like A Rat."
Portal started out as a student project called "Narbacular Drop." The game was about a princess navigating a dungeon by placing portals in order to escape. The game caught Valve developer Robin Walker's attention, who hired the students to rework the project as Portal.
Contributed by ORWELL6
At the end of Portal, Spoiler:when GLaDOS is disconnected and laying in the driveway, a hoop comes out of the air. This was VALVe's attempt at a new meme. Not knowing that "The Cake Is A Lie" would become the resident Portal meme, Hoopy the Hoop was lost into obscurity. The developers consider this the greatest disappointment in Portal.
Contributed by WilbosWorld
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 that was discovered recently called 'Portal clipping'.
Contributed by DidYouKnowGaming
There's an unused texture file in the game's code referencing the film "300". It can be found in the game's data as "wet_floor_sign.vtf."