Ever since Meta Knight was revealed to be a playable fighter in Super Smash Bros 4, there were eight empty slots on the bottoms of the character pages on the official Super Smash Bros. for 3DS and Wii U website. This is rumoured to mean that there are eight more characters left yet to be revealed. This rumor has spread around the Super Smash Bros. Series community leaving some confused, some thinking it's false, and some thinking it's true.
Who Started It
Someone named Jenny (NNID: whatya-lookin-at) made a post saying that there was no name for the eight slots rumour and she decided to call it "The Mysterious 8". It spread around pretty quickly and so, that's how The Mysterious 8 rumour came to life.
However, the rumor was proven false. Shulk was revealed to be a playable fighter a few weeks later. Then, 12 more characters followed and were revealed. (Being Ness, Falco, Wario, Lucina, Dark Pit, Dr.Mario, R.O.B, Ganondorf, Mr. Game and Watch, Bowser Jr, Duck Hunt and Jigglypuff). The Mysterious 8 post was deleted after Jenny was proven wrong.