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. There was never any intention to make these areas accessible in any way.
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).
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.
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.