While playing Zombies on the map "Five", it is possible for JFK to say after killing a zombie, "And I Thought they smelt bad...on the outside!" This is a reference to the line originally spoken by Han Solo in the film Star Wars Episode V: The Empire Strikes Back.
Contributed by Boyobmas
At the Main Menu where you're strapped in an interrogation chair, if you tap both the trigger buttons repeatedly you will actually get up from the chair and be able to walk around the room. You'll even get the Achievement/Trophy "Just ask me Nicely" for doing so. In the room there is also a computer terminal which you can use to read letters, view concept art, listen to the game's soundtrack, unlock "Dead Ops Arcade" and even play the 1980 text-based adventure game ZORK (Which you will also get an Achievement/Trophy for doing).
Contributed by Boyobmas
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The multiplayer map 'Nuketown' was inspired by the nuclear test site from Indiana Jones: The Legend of the Crystal Skull.
Contributed by Boyobmas
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During the Final Mission, one of the soldiers helping you is named "Sgt. Pepper". This is a reference to The Beatles' album: "Sgt. Peppers' Lonely Hearts Club Band"
Contributed by Boyobmas
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The mailboxes of the houses in Nuketown have the names Mason and Woods, referring to the campaign protagonists Alex Mason and Frank Woods.
Contributed by ISayWynaut
There are unused sound files of Hellhounds as well as character quotes that can be found in the files for "Five".
Contributed by frankfrost2
There is a cut perk for the Zombies mode called "Candolier". It was most likely going to let players carry extra magazines, like the multiplayer perk Bandolier.
Contributed by frankfrost2
Even though the Wunderwaffe DG-2 cannot be upgraded in Call of the Dead, the Wunderwaffe DG-2 JZ is still scripted into the game, and can be obtained with console commands. As a result of not being finished, it does not have a texture, and only appears white.
Contributed by frankfrost2
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In Nuketown, the purple house outside the map's boundaries is a replica of the house from the TV show "The Brady Bunch".
Contributed by DeerBoarDude
The codename for the zombie map Moon is "zombie_paris" in the console version of the game. According to Treyarch, the final map was going to be in Paris, but it was decided to be changed to be on the Moon and in Area 51 as they wanted to "go out with a bang."
Contributed by DeerBoarDude
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DLC maps can be found in the game's code on the disk. These are Berlin Wall, Hotel, Munich (later renamed as Stadium) Discovery and Golf Course (later named as Hazard).
Contributed by DeerBoarDude