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The license plate on every vehicle in Dead Island reads "D34D 1574ND" which translates to "Dead Island".
Contributed by ISayWynaut
The unlockable skill "Gender Wars" for the character Purna was referred to during development as "Feminist Whore". The original name along with alternate names for some of her other skills, which were "TeamSpirit1Purna", "SharpApprenticePurna" and "FeministWhorePurna", were found in the game's code of the non-retail version which had been accidentally released onto Steam. Techland came out to apologize over the incident and told that the developer responsible for the inappropriate names would "face professional consequences for violating the professional standards and beliefs Techland stands for."
Contributed by DidYouKnowGaming
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The logo on the game's cover of the North American release was altered to feature the Zombie walking upright instead of hanging from the tree. This was due to the Entertainment Software Ratings Board's (ESRB) strict guidelines about the types of content which is considered appropriate to feature in advertising and marketing materials. The logo did however remain unaltered in-game.
Contributed by KnowledgeBase
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After starting a new game, if you choose to turn left instead of right as the game tells you to, you will see another hotel door open. Inside the room are the couple from the first Dead Island trailer, in which the couple fought off zombies to protect themselves, and their daughter, who tragically turned into a zombie during the fight.
Contributed by PheonixGRX
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"Jason Voorhees" from "Friday the 13th" makes an appearance in the game. He carries his machete, and does 7 times the normal damage of enemy attacks, has a significant amount of health, and takes reduced damage from the player's attacks. Jason's eyes will also continue to follow the player around even if he is dead.

Inside his house the player can find a chainsaw as a usable weapon.
Contributed by Skarro