The ESRB leak is a leak of the content of Super Smash Bros. 4 that occurred several weeks prior to the Japanese release of Super Smash Bros. for Nintendo 3DS. It is a collection of photos and videos taken seemingly of a content evaluation video of the game sent to the Entertainment Software Rating Board. Unlike the equally-famous Gematsu leaks, the ESRB leaks were altogether true, and managed to leak the base game's entire roster, numerous stages, and several other details in regards to the modes and menus.
On August 19th, 2014, an anonymous user on 4chan's /v/ image board posted 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 parts of the leak would.
In the same thread, a user by the name of Motherfucking Leaker posted two more images. One showed renders for Ness and the Duck Hunt dog, also revealing a duck partner for the dog, while the other showed the entire roster. These two images were the subject of much debate: several users were doubtful that the characters' names would not be present in their icons, particularly so as they were on the images of the roster shown at E3, 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. Others adamantly denied that this was the game's base roster, citing both the perceived strange character choices and alignment of the rows towards the bottom.
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 widely mocked 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 the debate continued, more and more perceived errors were found in the images; combined with MasterLinkX admitting that he had photoshopped the images to halt the leak's spread, general consensus became that the leak was a very elaborate hoax.
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; many took this as confirmation that the leak was real, as copyright takedowns on rumors are traditionally done only in the event of an actual leak. Later that day, another set of screenshots was uploaded to imgur, featuring various menus, Classic Mode screenshots, and trophies.
Following this, a number of Miiverse posts from Masashiro Sakurai's "pic of the day" series featured menus matching those 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.
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.
A notable inconsistency was later discovered: contained among the leaked images was a trophy of the Fire Emblem character Tharja and a trophy count of 686. The base game lacks such a trophy and features 685 trophies instead. 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, one can infer that Tharja's trophy was removed to prevent the game from receiving a higher content rating. This could also be related to Palutena's censorship, which Masahiro Sakurai himself went over in a 2012 Famitsu column .
The later announcement that Mewtwo would be available as downloadable content is an interesting final note to the leak's choice to label it with a "?". Some believe that the leaker knew of the DLC plans, or that Mewtwo was a late cut from the roster, and the "?" was a reflection of that. However, it is far more likely that the leaker included the "?" over Mewtwo's image to mislead the community.
The leak remains the largest completely true leak in Super Smash Bros. history.