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Donkey Kong - Did You Know Gaming? Feat. JonTron
 
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An unused Ikegami Tsushinki Co., Ltd logo can be found along side the title graphics in the game's data. ITC developed the hardware and program code for the Arcade version of the game.
Contributed by ProtoSnake
The tune that plays while you have the hammer is based on the "Charge" bugle call that plays when a cavalry or troops are about to rush towards their enemies.
Contributed by Smirkytrick
If the "INTEND" part of the "NINTENDO" string (the data used to display the name) is altered in anyway in the game's code, the game will freeze after approximately 4 seconds. This is a form of copyright protection used to protect the game's data.
Contributed by xianc78
There are five unused audio files within the game, An alternate Pauline theme, two unused pieces for cutscenes, and two voice samples of what appears to be Pauline saying "Thanks" and "Help". These sounds weren't discovered until 33 years after the game's release.
Contributed by pyoro64
The twenty-second board is the final level of the game; Mario instantly dies within eight seconds of playing in the level, regardless of how many lives the player has left. This bug, known as a kill screen, happens due to a programming oversight which causes the game to set an impossibly short time limit.
Contributed by UltraMario3000
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Donkey Kong started off as a game called Radarscope. Radarscope was a success in Japan, but failed to sell successfully in North America. They reused some of the unused arcade machine in order to avoid any financial loss.
Contributed by GamerBen144
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The NES box art uses an altered version of Mario's sprite from Super Mario Bros. instead of Jumpman's in-game sprite.
Contributed by Boyobmas
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The Game & Watch title Donkey Kong Circus, released 3 years after the original Donkey Kong, may be set as a prequel to the original Donkey Kong. It is said that it was Mario who unwittingly started the conflict by mistreating his pet, and the scene depicted in Donkey Kong Circus may be an example of this mistreatment, if Mario is taming him to perform rather than being a mere audience member. However, the relationship between the two in this instance seems to be good-natured.
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In the NES port, it's possible to climb down the bottom right ladder and loop to the top of the screen, bypassing most of the stage.
Contributed by GhostLight
The game originally didn't have a jumping mechanic. It was implemented as a way to avoid obstacles when Shigeru Miyamoto and his team thought "If you had a barrel rolling towards you, what would you do?"
Contributed by CuriousUserX90
In the arcade versions of the game, there's a hidden message in the files:

CONGRATULATION !
IF YOU ANALYSE
DIFFICULT THIS
PROGRAM,WE WOULD
TEACH YOU.*****
TEL.TOKYO-JAPAN
044(244)2151
EXTENTION 304

SYSTEM DESIGN
IKEGAMI CO. LIM.

While the first three lines were overwritten with other data in the American build, the rest survived unchanged.
Contributed by DeerBoarDude
Miyamoto originally wanted to do a Popeye game, but couldn't get the license. He would later use the characters as inspirations with Mario taking the role of Popeye, Pauline as Olive and Donkey Kong as Bluto.
Contributed by Bean101
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In the Atari 400/800 port of Donkey Kong, you can view the programmer's initials (LMD) if you do the following:
• Play a game and set a highscore of x37 xxx, x73 xxx or x77 xxx
• Get a game over, with your last death being a long fall
• Set the difficulty to 4 by pressing Option three times
• Wait through the demo screen until the initials appear
This was not discovered until 26 years after the game was released, a year after LMD announced its existence.
Contributed by Mario-Fan
When Donkey Kong became popular, Nintendo was sued by Universal. Universal claimed Donkey Kong was a rip-off of King Kong. Lawyer John Kirby proved that Universal didn't even have trademark rights to "King Kong", and so they lost the case. Nintendo gave Kirby a boat named Donkey Kong and exclusive rights to the name Donkey Kong for sail-boats as a strange way of thanking him. It's also rumored the character Kirby was named after John Kirby, but this was never confirmed by Nintendo.
Contributed by DidYouKnowGaming