Urchin Underpass has a poster of a Blooper in a far off corner. Early in development, it was suggested to make the game a Spin-Off of the Mario series if the team was unable to come up with unique characters of their own.
The squid featured on the "Squid Beatz" cover resembles early concepts for the Inklings.
Dueling Dubs: The US translation for a lot of the Squid Sisters' dialogue can differ enough between the European/Japanese versions that they occasionally contradict each other:
In the EU, Marie still has her old worker's outfit from Walleye Warehouse. In the US, she burned said outfit.
The French version has Callie prefer sausages and Marie prefer marshmallows, the exact opposite of their opinions in the third US Splatfest.
Marie's hot dog shtick is completely absent overall in the European version, as she seems fine chowing down marshmallows. In fact, both her and Callie are seemingly obsessed with potato chips (or french fries in American English), which are mentioned exactly once in the American script, instead. Marie originally loved marshmallows in the North American version, however due to the Hot Dog vs Marshmallow Splatfest that has since been retconned.
Dueling Works: With Sunset Overdrive, the other wacky, colorful, Real Is Brown-averting third-person shooter that released around the same time, which was also a console exclusive (Xbox One) — albeit one with a single player focus rather than Splatoon's mutliplayer one. Both games received solid review scores, but Splatoon eventually sold four times as many copies as Sunset Overdrive despite being on a console with a much smaller install base, and started a franchise that is now a Killer App for Nintendo systems. In contrast, Sunset Overdrive saw little discussion in the years since its release and remains a standalone game despite the Sequel Hook in the game's ending.
Easter Egg: The two statues in the Museum d'Alfonsino can be heard faintly laughing at night. This is now only accessibly by hacking into the game, as Splatfests ended after July 2016.
Despite originally stating that major content updates would end on January 2016, the producer of the game went on record saying that more new content was a possibility in the future. Indeed, a March 2016 update confirmed that more weapons would be added to the game, with the last of these weapons being added that June.
Genius Bonus: A Sunken Scroll hidden in one of the Octo Valley levels contains sheet music, stating it's an ancient anthem that all young Inklings know. People who can read sheet music will realize later Agents 1 and 2 sing this song in the final stage against DJ Octavio.
I Knew It!: Many had speculated that when the Splatfest events would end, the final theme would likely be "Callie versus Marie". This was especially the case after the Squid Sisters got in a serious argument after one of the NA Splatfests, with datamining revealing that they would have fought with each other regardless of who won. This was confirmed on June 30th 2016, with Nintendo announcing this to be the case.
In North America, Gamestop purchases included Inkling-themed Mii Fighter costumes for Super Smash Bros. for Wii U, and Target preorders came with a toy water gun based on the Splattershot. Preorders at EB Games in Canada included a special double-sided poster.
Europeans who preordered from GAME got a beanie resembling a orange female Inkling's hair, and preorders at Gamestop granted a T-shirt.
Preordering from Amazon Japan netted you a sticker sheet.
Preordering on the Nintendo eShop provided an extra costume for use in the pre-release "Testfire".
Sleeper Hit: The game was advertised a bit during E3 2014, but didn't garner a lot of interest and faith until the several Testfires and commercials massively boosted its presence. Before the end of its first month, it managed to sell 1 million copies worldwide; this was a greater shock in Japan, where shooter games don't prosper anywhere near as well as they do in Western markets. Also keep in mind the game was both exclusive to a struggling console and had yet to receive any content updates at that point. After a year, it had sold well over 4 million copies, with 1.36 million sold in Japan alone: enough to make it the most successful new console IP in that region since Wii Sports.
Urban Legend of Zelda: Octolings were a common source of rumors, mostly the idea of them being playable in a later update. The most popular rumor was them coming out in October 2015, due to the pun potential and the fact that Japan was running a literal "squid vs. octopus sushi" Splatfest at that time (even the Japanese SplatNet had an "Octo" theme during that Splatfest), but nothing came of it, and by October 2016 development had shifted over to Splatoon 2. A lot of this stemmed from the fact that, on a technical level, playable Octolings are possible in OG Splatoon - they are, of course, really just Inklings with different face, eye, and "glove" textures and a unique hair model, and so they animate and function perfectly well as player entities. Particular fuel for the speculation engine came from the fact that the Octoling eyes, and specifically their unique eye lining, are textured in-game despite their eyes never being visible during normal gameplay and the fact that the Octoling hair actually is texture-rigged to be fully functional with the squad ink system, despite the hair only ever appearing in two colors when encountered in Hero Mode. Data miners also noticed that the Octoling-specific meshes did get patch updates during the game's year-long update cycle. Ultimately, though, Octolings in Splatoon 1 can only be played via game save modification using third party tools (and the Big One was never solved - the Splat 1 Octoling hair does not play nice with most hats, as it turns out) and Nintendo, to this day, is extremely aggressive in nuking your account if you are detected with such a mod online.
The Iwata Asks for the game goes into more details about the character creation process. The initial prototype used black and white cubes that the dev team jokingly referred to as tofu. When it came to actually create proper characters to use for the game, they began briefly considered human characters (at which point Mario was given as an option), before choosing anthropomorphic rabbits that burrowed into ink so they could use the ears as position indicators. This got a muted response from others around the company, which led to them taking a second look at all the game mechanics they wanted to implement and realizing humanoid squids would be the best option. Even then, they were more like Funny Animal squids before they settled on characters that could transform between squid and human.
An interview with Famitsu on July 2015 revealed that the Squid Sisters were at first going to be called "Shrine Maidens", and their role was to transmit divine messages from god to the populous, with said messages coming from a fax machine. This last element is still present in the game, with the Squid Sisters still receiving Splatfest themes from a fax machine decorated like a shrine. The fax machine was also meant to have a greater role in the news during that stage of development.
The same article reveals that the heavy emphasis on the Inkling Girl in promotional materials was partly the result of the devs wanting to add another female lead character to Nintendo's roster, to the point where they seriously considered making Inklings an all-female race.
Regarding the final boss, he was going to be a wasabi chef at first, before turning into the DJ he is in the final game.
The game files reveal plenty of Dummied Out gear that are either slightly different from their final forms, or go unused entirely. Of note is the "Warrior Headdress", a Native American war bonnet that was likely cut due to Values Dissonance.
There was an Octarian enemy scrapped from single-player, and the mode originally gave the players five lives instead of three.
Dataminers were able to uncover alternate Squid Sisters dialogue that would have been used if the other team had won from almost every North American Splatfest from "Art vs. Science" onwards.
Krakens were originally named "King Squids", Seekers were "Bomb Chasers", and Echolators were "All Markings".
Concept art for the Squid Sisters show off revealing outfits that showed off a lot of skin and accentuated their legs.
The E3 2014 reveal trailer showed earlier versions of maps in the games. They appear heavily altered in the final product, with several being significantly larger at that stage of development. It also showcased a different version of the Inkstrike special with a larger and spherical blast effect instead of the relatively focused vortex of ink it became by the release.
The E3 2014 demonstrations of the game were done on a very early build with only one map done, and even then it was incomplete. Videos of these demos feature several differences from the final game:
There were originally two rounds per match instead of one.
The music in general was very different. "Splatattack" in particular used different vocals and began with the chorus, with this version of the song later released as a bonus track on the OST. The one-minute left music was entirely different to the song "Now or Never" that is featured in the final version.
The Splattershot variant in the demo had the Splat Bomb for its subweapon instead of the Burst Bombs, and had no Special.
The D-pad only stated "signal", and it was obviously used for that purpose.
You received points for splatting others. note The amount of points were actually still based on the turf covered by the explosion of the opponent being splatted. Later builds removed the display of said points.
The tape at the end of a match was red instead of yellow.
Instead of being separated by team, the Results screen was leaderboard-style and was in order of most points to least.
The intro animation to the match was slower, more clearly showing the player's transformation from squid to kid.
Averted with the manga. It has been localized outside of Japan.
Played straight with the anime. There is no English dub or official subbed version.
Urban Legends: Due to translation issues, prior to release most international fans (and news outlets) thought the animated manga was a full-fledged anime. When it turned out to just be the manga voiced-over, fans either thought that CoroCoro had lied to them or that they were just testing out the series' reception to see if it should get a "real" Animated Adaptation.