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Metroid - Did You Know Gaming? Feat. WeeklyTubeShow
 
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If the player enters "ENGAGE RIDLEY MOTHER FUCKER" in the password menu, the game will begin with many game breaking quirks such as garbled graphics, the game being unable to scroll, unable to turn into a morph ball, and the total playtime being registered as 2755656 hours (314 years). Entering this password can also cause the game to crash at a black screen.
Contributed by SkyminHAZBOZ
There is room inside of the game's cartridge for a battery, meaning the game would have been capable of saving games instead of using a password to resume. It is unknown why this was never implemented.
Contributed by KidDivinegon
There are three inaccessible rooms in the game's code. All of these rooms are in Norfair, and are copies of other rooms in the game.
Contributed by danjoredd
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The Japanese Famicom Disk System version features a File Select screen with three save slots, like The Legend of Zelda. You can also see how many hours you have spent on your mission, where one hour of gameplay represents one "day." The NES version uses a password system instead.
Contributed by ClaudX
Before being changed to Metroid, the game was going to be named Space Hunter. Also, Space Hunter was used as Samus' title instead of Bounty Hunter in the original game manual. There wasn't enough time left in development to change the occupation, so Space Hunter was left in the original Instruction Manual.
Contributed by Pogue-Mahone
The jingle heard when the player picks up an item is similar to that of "Ys I: Ancient Ys Vanished".
Contributed by pAroxizm
Samus was originally planned to have unique sprites and animations for whether she was facing left or right. These sprites are still stored within the game's ROM. This animation idea would eventually be seen in Super Metroid.
Contributed by MightyKombat
On the box, it claims that the game is a "two player alternating game." This is believed to be a misprint, possibly copied from a template.

The first multiplayer game in the series was Metroid Prime 2: Echoes, although Samus did appear in the multiplayer Super Smash Bros. (1999) and Super Smash Bros. Melee (2001) prior to to the 2004 release of Echoes.
Contributed by gamemaster1991
Yoshio Sakamoto, co-creator of the Metroid series, has stated that Metroid was influenced by the 1979 sci-fi movie "Alien". This is evident when considering the following:
• Both Metroid and Alien feature a female lead protagonist.
• The Nostromo's computer is referred to as "Mother" and the antagonist of Metroid is Mother Brain.
• The Xenomorph aliens and Metroids both have multiple stages of development, and the first stage of both species latch on to other creatures.
• The boss "Ridley" is likely named after Alien's director Ridley Scott.
Contributed by DidYouKnowGaming