A few hours into Wolfenstein: The New Order, once you awaken in 1960 and escape your captors, you'll eventually make your way into the Resistance movement. It's not too hidden but there's a wonderful easter egg in this building.
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Head on up to the very top of the underground fortress the Resistance inhabits and you'll find a bed on the floor. Try to sleep on it and it will warn you that you'll have a nightmare there, and you'll have to hold down the button to attempt it.
A poster on the wall for Wolfenstein 3D suggests that this might be the kind of nightmare you might want to have, and indeed, once you jump into it you'll find a faithfully recreated first level of Wolfenstein 3D. You'll still control the weapons from The New Order and all of the ammo boxes and armor shards are updated, and everything is all shiny and HD, but this is the complete old level, pixelated soldiers and all.
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Even the secret walls that reveal hidden levels when clicked are intact. Check out the video below and enjoy!