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In test chambers 2, 6, 7, and 9 in the Portal add-on, it is possible to briefly spot Doug Rattman in various areas before he runs away.

In the Portal series, Doug Rattman is a scientist who worked for Aperture Science and is an integral part of the series lore. He is alluded to but never actually seen in the first two Portal games (though he did star in a promotional comic for Portal 2). This is technically the very first time players have physically seen Rattman in any official Portal game.

These Easter eggs were not discovered until over a year after the game had launched.
Contributed by Boyobmas
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In the Doctor Who world, one of the comments the pedestrians will make is "I went to this GREAT pie shop. Used to be a barbers, y'know...", a reference to the Victorian penny dreadful character Sweeny Todd, the Demon Barber of Fleet Street.
Contributed by KnowledgeBase
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If the console's internal clock is set to any fourth date of the month, i.e. 4th, 8th, 12th, etc. the TARDIS interior will feature the Fourth Doctor's alternate TARDIS interior.
Contributed by KnowledgeBase
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In the opening cinematic, the number 42 splashes from the surface of Foundation Prime. The number 42 is a reference to The Hitchhiker's Guide to the Galaxy series where the number 42 is the "Answer to the Ultimate Question of Life, the Universe, and Everything".
Contributed by KnowledgeBase