In the days before the internet, urban legends and straight-up hoaxes spread like wildfire, thanks to an overeager willingness to believe anything and nothing to keep the rumors in check. There was no one to deny the existence of wild claims like bringing dead characters back to life, with the exception of a few gaming magazines (such as EGM, which as you'll see often perpetuated the hoaxes themselves). And when they turned out to be untrue lies, the harsh reality of our crushed dreams stung. But you know what stung more? TRYING TO BEAT MORTAL KOMBAT 100 TIMES IN A ROW WITHOUT SUFFERING ANY DAMAGE IN ORDER TO UNLOCK THE SONYA BLADE NUDE CODE. Ahem. These are the 7 cruelest hoaxes and urban legends in videogame history.
Mario and Luigi go together like spaghetti and meatballs, like donuts and coffee, like mushrooms and fire flowers so it wasn't unreasonable to assume Luigi would be a playable character in Mario's first foray into the 3rd dimension. Sure, Nintendo had specifically said it was a single-player affair, but that didn't mean whoever was playing couldn't choose between Mario and Luigi, right? Why would Nintendo leave its 2nd most recognizable plumber out of the game entirely, when even Yoshi makes an appearance?
That was the attitude the internet seized upon at the release of Super Mario 64 (note it wasn't "Super Mario Brothers 64"). Luigi had to be there somewhere. Why would Mario leave his taller, weirder brother behind? The first "clue" the internet found was "L IS REAL 2041" (or something like that) illegibly scrawled on a statue: somehow, this meant Luigi was real and in the game (other theories included promotion for Paper Mario). Coming in with the assist were numerous texture hacks, photoshopped screenshots, and tales of playing as the green plumber flooding the internet.
Shockingly, he was not in the game. But then Nintendo released the Super Mario 64 remake for the DS and he was included along with Yoshi and Wario. It'd be nice if every rumor was eventually validated with a handheld remake.