ESRB leak
The ESRB leak is a popular leak that occurred several weeks prior to the Japanese release of Super Smash Bros. for Nintendo 3DS. Unlike the equally-famous Gematsu leaks, the ESRB leaks were altogether true, and managed to leak the entire roster of the base game and numerous stages.
Screenshot leak
The leak started out on August 19th, 2014 when an anonymous user on 4chan's /v/ image board posting a list of all previous characters in the Super Smash Bros. series and all of the revealed newcomers for Super Smash Bros. 4, and asked other 4chan users who they thought would be cut. Later, another user made their own post with the previous user's photo modified to include never-before seen icons on top of unconfirmed veterans Jigglypuff, Ness, Dr. Mario, Mr. Game & Watch, Wario, and R.O.B., as well as four added to the end with Bowser Jr., the dog from Duck Hunt, Dark Pit, and Shulk. The Ice Climbers, Lucas, Solid Snake, Pichu, Roy, Young Link, Squirtle, Ivysaur, and Wolf all had red X's added to them, while Mewtwo interestingly had a ? put over its face, which, given that the leak seemed to originate from the ESRB, may have been included as speculation or to mislead readers, as many later parts of the leak would.
In the same thread, a user by the name of Motherfucking Leaker posted two more images. One showed the Duck Hunt dog and Ness's renders, also revealing a duck partner for the dog, while the other showed the entire roster. These two images were the subject of much debate, as several users were doubtful that the characters' names would not be present on their icons, as they were on the officially shown images of the roster, and other users compared Ness's render to that of a previous rumored leak about his confirmation, which had been proven false almost an entire year prior.
A third account later posted five more images which, respectively, featured Shulk's render, the stage select screen, Dr. Mario's render, the Customs menu, and another shot of the roster including the bottom screen, which revealed Bowser Jr.'s render and the fact that he would fight in his Junior Clown Car, and also gave Player 1 the name "ESRB0083", confirming the leaker's connection to the ESRB. Once again, these five images became the matter of some debate - several users compared Dr. Mario's render to a popular fan-made render, and other users claimed that Shulk's render was an edited Little Mac, a claim that became popular in the weeks following the rumor's confirmation. Despite the doubters, the leak picked up several believers and continued to pick up speed.
The 4chan thread was eventually closed, prompting a leaker with the account name of MasterLinkX to start a thread on GameFAQs, with three more images. The first featured a battle between Charizard, Lucina, and leaked characters Ganondorf and Shulk on Yoshi's Island. The second showed another shot of Shulk's render on the victory screen, and the third was a shot of the top screen from the same moment, showing a clearer angle on Shulk's model and one of his victory poses. As had become the norm, these screenshots were debated, and as more and more errors were found in the images (which MasterLinkX stated that he had photoshopped in to try to stop the leak from spreading), more and more followers started to doubt the leak's legitimacy.
Video leaks
As the leak started to lose popularity, believed to have been disproven by a large portion of the community, a YouTube user by the name of Izat True posted multiple low-quality videos of the previously-pictured battle between Charizard, Lucina, Ganondorf, and Shulk on Yoshi's Island, as well as Bowser Jr., Bowser, Kirby, and Pikachu battling on Battlefield. However, later that day, Nintendo of America removed all of the videos for copyright reasons. The videos were later reuploaded on various different accounts. Later that day, another set of screenshots was uploaded to imgur, featuring various menus, Classic Mode screenshots, and trophies.
After the pictures and videos stopped being posted, a number of Miiverse posts from Masashiro Sakurai's "pic of the day" series featured menus matching those pictured in the leak, and Shulk himself was confirmed on August 29th, 2014 through a Japanese Nintendo Direct, with his moveset, animations, and render matching what was shown in the videos and screenshots. Furthermore, a demo of the game was released that confirmed the presence of several perceived oddities, such as Yoshi being mixed in with the Mario characters and the lack of names on the character select screen.
Aftermath
The leak was definitively confirmed when the game was released in Japan and the entire roster of characters and stages was released by players who had received it.
However, an inconsistency was later discovered - among the last set of screenshots, released alongside the videos, were pictures of a trophy of the Fire Emblem character Tharja - a trophy which was not present in the final game. In fact, the leaked photos showed the total trophy count as 686 rather than the 685 non-DLC trophies present in the final game. Given that the trophies featured in the leak contained possibly objectionable content or names (such as Tharja's skimpy outfit or the Devil Car) and that the leak consisted of material evaluated by the ESRB, it is reasonable to infer that Tharja's trophy was removed from the game to prevent raising the game's content rating.
The leak remains the largest completely true leak in Super Smash Bros. history.
Gallery
The picture revealing Duck Hunt's render and duck partner.
The entire roster. This picture also revealed Bowser Jr.'s render and the usage of his Junior Clown Car.
Shulk's render, with a comparison to Little Mac's face from a screenshot.
Dr. Mario's render.
Shulk, Lucina, Ganondorf, and Charizard battling on Yoshi's Island.
The victory screen and Ganondorf's render.
One of the trophies featured in the last group of screenshots that showed pertinent content submitted to the ESRB. These screenshots depicted trophies that include religious references (e.g. Demon King Arzodius, Devil Car), suggestive content (e.g. Candy Kong, Pauline), and crude humor (e.g. Wario, Specknose).