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According to Seamus Blackley, the creator and designer of the Xbox, there is one more easter egg in the console that has yet to be found as of November 2021. It is related to the Xbox's boot animation and he believes that it cannot be accessed through certain controller inputs during the animation.
Contributed by MehDeletingLater
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In 2021, an anonymous Xbox developer informed Kotaku of an undiscovered developer credit on the Xbox. To find it, insert a music CD, and from the Audio CD screen choose Copy, Copy, and then New Soundtrack. Delete the default soundtrack title and replace it with the following name (without quotation marks): "Timmyyyyyyyyyyyyyyyyyyyyyyyyyy!"

Wait for the CD to finish ripping and then return to the main menu. Now going to System Info within the Settings will reveal an updated screen crediting the four programmers that made up the Xbox Dashboard Team.
Contributed by MehDeletingLater
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A lengthy hidden developer credit for all those who worked on the Xbox console can be found by inserting a music CD, going to the Audio CD menu, selecting and pressing Copy, Select All and then Copy again. You are then taken to a menu to name the Soundtrack you have just selected, where you should enter the following name without the quotation marks: "[two less-than symbols]Eggsßox[two greater-than symbols]"

When you hit Done, the Xbox loading screen appears and will take you to a credits sequence, thanking the developers who made games for the Xbox and their playerbase before giving thanks to The DirectX Team, Microsoft Hardware, Worldwide Operations, The MSN Gaming Zone, Windows Core, and the entire Xbox Team and their families for tolerating the long work hours it took to create the console. The credits end with "XBOX 2001".
Contributed by MehDeletingLater
Pressing X and Y at the same time while playing music on the Xbox's Audio CD player will display colorful visualizations to complement the music.
Contributed by MehDeletingLater
After the console has been on for a while, obscured noises can be heard over the top of the normal ambient background noise. Microsoft stated that they are actually tweaked and modified public domain sounds from the Apollo days of NASA.
When the original Xbox was made, Microsoft lost $125 per unit sold. After 4 years, Microsoft had lost $4 billion.
Contributed by Botahamec
Sega's then president, Isao Okawa, wanted Microsoft to include Dreamcast backward compatibility in the Xbox, but negotiations fell through when Microsoft refused to include online play for Dreamcast games.
Contributed by xianc78
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The Xbox controller was redesigned to appeal to Japanese gamers because Microsoft had trouble selling the Xbox to a Japanese audience. In the end Microsoft came up with the Controller S, an Xbox controller which was more compact and attractive to the Japanese demographic. This design would eventually be adopted in America as well, and a decade later became the standard.
Contributed by Makarov
The original Xbox prototype was built using dismantled Dell laptops.

"In 1998, Berkes and his team ordered a few Dell laptops, took them apart and built the first prototypes of a Windows gaming console.

Ed Fries was leading Microsoft's games publishing business when the four Xbox founders pitched a "Direct X Box" based on the Windows DirectX graphics technology that was developed by Berkes' team."
Contributed by DidYouKnowGaming
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The Xbox was originally proposed as the "Direct X Box", because it utilized Microsoft's DirectX technology. It was later shortened to Xbox.
Contributed by ThisIsGamer