Forty years ago, the series' original game was released by Nintendo. As well as trying to complete it, fans were desperate to access a secret world that became legendary.

Today, video gamers are always looking for the unexpected when they sit down to play. All the biggest titles feature easter eggs, secret areas, intentional "glitches" and other surprises woven into their expansive worlds: see, for example, Ubisoft's 2011 racing game Driver: San Francisco, where driving a Delorean (of Back to the Future fame) at 88mph will unlock a secret level called Blast From the Past, and The Sims 4 (2014), where typing in "motherlode" as a "cheat code" will instantly add a flood of money to your budget.

But the history of these unpredictable elements of gameplay goes all the way back to 1985. Nintendo's Super Mario Bros, the first game in the feted Super Mario series, was released on its flagship Nintendo Entertainment System (NES) 40 years ago this month. And a genuine error made its way into it. When coding the game, developers made a mistake that captivated gamers at the time and subsequently influenced the whole history of video game development.

The glitch came early on in the game, and involved making the protagonist Mario – an Italian plumber tasked with saving a princess from a castle – jump at a particular wall at just the right angle. If you got it right, he would glide through the wall and into one of the Warp Zones: the areas through which Mario could transition between levels by jumping down a pipe. But if you sent him down a pipe in this Warp Zone, he'd arrive in a place that wasn't supposed to exist – an underwater level that looped back on itself to infinite effect. The only way out was to reset or die.

This coding error occurred in World 1 level 2, which was introduced on screen as "World 1-2". After Mario had glided through the wall and down the pipe in World 1-2, text would appear on the screen to indicate which new world and level he was entering, as per usual. But thanks to the glitch, there was a blank space where the world number would usually be, so it read as "World -1". This secret level therefore became known as the "Minus World".

Rumours of the phenomenon spread through word of mouth soon after the game's release, before entering gaming folklore in 1988 when American magazine Nintendo Power reported on it. "Explore the mysterious minus world," said the article, teaching readers how to access the hidden level. It's now regularly celebrated as "the greatest glitch of all time", but upon its discovery many questions arose: did Nintendo put the level there deliberately? Were there other secret levels in Super Mario Bros? Were there other secrets hidden in other games?

Nintendo's legendary auteur Shigeru Miyamoto denied that the Minus World was a deliberate feature of the game. But it added to Super Mario Bros's allure of unpredictability: here was a game whose gradually unfurling world was unlike anything that came before it.

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© BBC