The second boss of the game, Ridley, was named in honor of Ridley Scott, the director of Alien (1979), as the story and atmosphere of Metoroido (1986) were heavily inspired by this famous science-fiction movie.
Some gamers stumbled upon a glitch in the game, where it was possible to jump through a door and get stuck in a wall. From there, it would be possible to jump through walls into normally inaccessible areas with strange layouts. This gave rise to a popular video legend of the "Secret Worlds" in Metroid, which were only accessible to clever players. Years later, it was revealed that these secret levels were actually unused map data that were supposed to be outside the playable area. Just like the infamous "minus 1 level" from Super Mario Bros. (1985), there was never any intention to make these areas accessible in any way, so the only way out of them is by resetting the console.
In this game, getting a beam weapon replaces the beam weapon you had previously, which can lead to some problems later in the game. This would be later corrected in Super Metroid, in which beams could be combined.
The creation of Samus Aran was inspired by the Ellen Ripley character from the Alien movie franchise. Ridley, one of the bosses from the game, was even named after the director of the first Alien film, Ridley Scott.
When it was first released, Metroid held a secret about its armor-clad protagonist, Samus Aran. When the game was beaten in under two hours, the player was presented with an image of Samus, whose armor then disappeared, revealing that the character is actually a woman. Samus Aran has since gone on to be recognized as one of the earliest and most famous female video game protagonists ever.
The Morph Ball was partly invented out of convenience. At the time, it was much easier to animate a ball rolling through narrow passages than a computer figure crawling on all fours. Samus crawling through narrow passages was finally displayed in Metroid: Zero Mission (2003).
The events of this game have since been retconned by the Game Boy Advance remake, Metroid: Zero Mission (2003). That game is narrated by Samus Aran as the 'true account' of what happened on her first mission.
There are bird-like statues throughout the game holding power upgrades for Samus' suit. It wasn't until Super Metroid (1994) that these were retrospectively identified in the manual as belonging to the Chozo, the original population of Zebes before the Space Pirates invasion. The backstory of this ancient race was later explored in Metroid Prime (2002), the remake Metroid: Zero Mission (2003), and several Metroid-related comic books.
The planet where the action takes place is called 'Zebeth' in the game's intro, and the correct pronunciation is 'Zeh-bess'. However, in the game's manual, the planet is referred to as 'Zebes' (usually mispronounced as 'Zeebz'). This discrepancy came from a communication error. In Japanese, the -th and -s sounds are pronounced the same, so both spellings would have been acceptable. However, a different spelling was unintentionally used during the translation of the manual from Japanese. This spelling was the one that stuck, and it has been used in all subsequent Metroid games and related media ever since.
The trivia item below may give away important plot points.
The decision to make the mysterious bounty hunter Samus Aran a woman was made halfway through the development process. Samus was initially going to be a male, but it was changed because it would be a cool surprise for the gamers. The instruction booklet that came with the game purposely avoided disclosing Samus' gender, even going as far as to suggest that the character was a cyborg.