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A presentation about financial competition between Aperture Science and Black Mesa can be seen in-game Spoiler:while exploring the Aperture offices just before the boss room with GlaDOS, setting the game within the Half-Life universe.
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In the observation rooms, the keyboards have the letters A, S, D, E, R, H, N, I, and P highlighted in yellow. Unscrambling the letters spells "Adrian Shephard" who is the protagonist of Half-Life: Opposing Force.
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Throughout the game, a heavily corrupted cake recipe will scroll across various computer monitors. On the second to last level of the game, a GLaDOS memory core will also recite the recipe. Many of the ingredients on the list are real Black Forest Cake ingredients, however there are oddball ingredients including rhubarb, polyester resins and many miscellaneous fish-shaped objects.
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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."
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