While Intelligent Systems is known for keeping upcoming Fire Emblem Heroes content under wraps until they are announced the public, every now and then the developers slip up and leak things earlier than intended. Such examples are all listed here, as the list continues to grow as the game goes on.
- Whenever an update is released, dataminers usually will sift through all the content included in it within an hour, including new heroes and their stats and skills, as well as any game assets and special maps planned in the future, and sometimes new characters introduced in the story. Sometimes, the updates will also include Heroes that are planned to be released days after the update, but before being officially revealed through announcements. One notable example is the Book II Chapter 13 update in mid-September 2018, where not only the "Nohrian Dusk" units were datamined, but also the "Generals of Múspell" units as well, which would have been released a week later.
- The art for the Christmas variants of Robin and Tharja were discovered almost a day after the game's launch in 2017, but they weren't officially released until December of 2017.
- For the few minutes it was up in early June of 2018, a preview image of the "Scattered Fangs" banner included Nino on a pegasus was released on the home screen within the game, only to be removed when the planned corresponding trailer wasn't released yet. This revealed to the players that the next banner was Fire Emblem: The Blazing Blade related, as well as showing who the next alternative unit was in that banner.
- For some odd reason, Hríd's voicelines were included in the "Fire and Ice" banner update in middle-to-late November 2018. Dataminers noticed this, revealing his release as a Hero (albeit a Legendary one) in late-November 2018. It also revealed in that same update that someone named "Hel" was included in Heroes, hinting players alongside The Stinger of both the November Feh Channel update and the Remembering Book I & Book II videos on the theme of Book III.
- Google Play leaked the announcement of a legendary version of Roy
days before the banner's trailer was scheduled to drop. - Dataminers managed to find the identities of the 3/8/2019 The Binding Blade heroes four days early by sifting through the resources of the "Meet The Heroes" website. The only character from the banner who managed to not get leaked was Thea.
- On Learn with Sharena in late May 2019, the "effective against dragons" status was shown. One of the skills for the "effective against dragons" status that can give it to allies included a skill called "Divine Fang"◊, which in the Japanese version is called "Divine Dragon King's Fang", hinting that May 2019's Mythic Hero would be Naga from Mystery of the Emblem and Awakening, which was proven true when the trailer dropped.
- A trailer for the first Summer 2019 banner was delayed due to a magnitude 6.8 earthquake that hit Japan prior to when the trailer was supposed to be released. The image for the banner itself, however, began circulating throughout the Internet, featuring summer variants of Gunnthrá, Laevatein, Laegjarn, and Helbindi. However, speculation on the banner's lineup began a day before the banner's trailer was planned to be released, as one of the bonus units for Aether Raids in late June 2019 was a placeholder silhouette with ? (likely to have been Summer Gunnthrá), which the developers later clarified was supposed to be the regular variant of Gunnthrá. Players would later correctly predict that she was going to be on that banner.
- For a brief period of time after the release of Mythic Sothis at the end of July 2019, a few pages into the notification board in-game leaked next month's 8% banner, revealing its lineup, including the Legendary variant of Julia, who would be the month's Legendary.
- In the Tips & Tricks: Changing Winds video in early August 2019, Petra was seen using an Atk Tactic Sacred Seal, which wouldn't be released until the "A Hero's Mettle" Tempest Trials in mid-August 2019.
- Images of hidden Google Play achievements are always a source of leaks. An example is in Early-September 2019, where one showed what resembles a green-haired figure with a tiara◊. Many players predicted that it was Nagi from Shadow Dragon and New Mystery of the Emblem, which was proven to be true when "The Chosen Ones" banner was revealed in Mid-September 2019.
- In late December 2019, when the update for Legendary Celica was released, it also included the 2019/2020 New Year's heroes as well, which included Eir, Anna, Lethe, Selkie, and Alfonse & Sharena, with the latter two as the Duo Hero for that seasonal event and the banner planned to be released a week later.
- Whenever dataminers find that an enemy-exclusive tag on a weapon is removed, it usually means that whoever wielded that weapon will be released soon. For example, in the "A Star is Born" update in mid-January 2020, alongside the Tokyo Mirage Sessions ♯FE characters and stats, dataminers found that Líf's Sökkvabekkr had the enemy-exclusive tag removed, hinting at his release as a Mythic Hero in late January 2020.
- In the March 2020 update, dataminers found two new status effects that had the armor and dragon icons on them, suggesting that these could nullify armor and dragon weaknesses, respectively. Just a few weeks later, Duo Spring Idunn and Fae were released, with a Duo ability that grants exactly that.
- On the "Meet the Heroes" site with Sharena in June 2020, four of the banner units (Mirabilis, Anna, Mustafa, and Say'ri) were featured. The latter three hail from Awakening, but one more Hero was also shown on the page that wasn't in the trailer or mentioned anywhere: Emmeryn, also from Awakening, who would be released as a quest reward unit.
- Update 4.6.0 in early June 2020 had assets with the "TwinWorld" tag on them, hinting at a new type of hero and game mode. It was later revealed in the June 2020 Feh Channel that it was related to "Harmonized Heroes" units, which are Heroes from different games paired up like Duo Heroes, and the "Resonant Battles" mode, where Harmonized Heroes score more.
- The trailer for the Summer banner in June 2020 was leaked about half a day earlier than was planned, as well as the accompanying June 2020 Feh Channel, revealing that Selena, Joshua, and Lute from Sacred Stones were on the banner, as well as the Harmonized Hero Mia & Masked Marth.
- Among the usual files within the "A New Future" banner update in August 2020, dataminers actually found something interesting, in that it was less of finding interesting files as it was more of finding a lack of certain files. Two things were not found in the datamine: Freyr's attack and special artworks, and his possible exclusive weapon. Since the banner trailer revealed that Freyr appears as an enemy unit, this was especially strange. Indeed, this later hinted at a major event in Book IV Chapter 10: Freyr taking Suicide by Cop to stop his sister Freyja.
- In the "Fire Emblem Heroes - Music Compilation" video released in late February 2021, alongside the music played, characters from the games matching the music are shown, alongside their attacking sprites and animations. The "Truth, Despair, and Hope (Sacred Stones)" section showed Regular Lyon with his Naglfar's sprite being slightly brighter and having a glow compared to the original version, indicating that it was a refined version like the other refined tomes shown beforehand, which Regular Lyon didn't have when the video was released. The Version 5.2.0 update announcement two days later revealed that Regular Lyon's Naglfar would get a refine in early March 2021.
- In the "Book V Mid: Nótt and More" update in early June 2021, dataminers found that the Hero Check introductions for Marianne from Three Houses were found connected for the first Summer Tempest Trials+ banner in late June 2021, hinting that the first Summer banner for 2021 would feature characters from Three Houses, which would include a Summer variant of Marianne. Sure enough, she, alongside other Three Houses characters, were revealed in the "Summer Vibrance" banner trailer in late June 2021, although Marianne was the assist character with the Summer variant of Hilda as a Duo Hero.
- In the 5.7.0 update in early July 2021, alongside the units and skills in the "Summer's Dream" banner, dataminers also found a character named "Nifl", the Goddess of Ice, as a Blue Dragon Cavalry unit and the boss of the "Ice & Flame 1" Tempest Trials+ event. This hinted at what the "Ice & Flame" Tempest Trials+ series would be all about.
- In the "Keepers of Faith" banner update in mid-August 2021, dataminers found a tome not belonging to any hero as of that time. Said tome has the symbol of Daein from Path of Radiance and Radiant Dawn on it, and Pelleas, who is from Radiant Dawn and was also announced as the Grand Hero Battle for that update, did not wield it. This proved to be a hint for the playerbase that the Legendary Hero of that month was Micaiah.
- The Halloween 2021 units were accidentally revealed on a tweet regarding Ótr's Mythic banner, showing that the next wave of Halloween units were Male Grima, Rhea, Kurthnaga, and Sothis. What the leak didn't reveal, however, were Male Byleth as Sothis' cohort, and the Tempest Trials freebie, which turned out to be Sophia.
- Downplayed, but the announcement for the Version 5.10.0 update was announced on the official Fire Emblem Heroes website about 8 hours before intended, revealing new Memento Events and a quality of life change in Heroes Journey, weapon refines for Owain, Bridal Tharja, Laevatein, Julius, and Soleil, and other minor additions and changes.
- A leak from Discord before the release of the Ninja Training banner in early November 2021 detailed the roster for upcoming characters, playable or otherwise, for all of November and even some of December for that year: variants for Igrene, Shinon, Male Corrin, Female Corrin, and Shamir, Resplendents for Serra and Ryoma, Thórr as the boss of the "Ice & Flame 5" Tempest Trials+ instead of Ascended Laegjarn, Volke, Astrid, Marcia, Tanith, and Ascended Laegjarn in the "New Heroes & Ascended Laegjarn" banner, with Bertram as the Grand Hero Battle, and Eitri being Promoted to Playable, all of which later came true with the release of those banners in November 2021 and the resulting datamines. The leak also hinted that Fáfnir was not going to be released alongside Eitri, but it did miss that Thórr was going to be released alongside Eitri in November 2021's Legendary and Mythic Hero banner. However, the leak of a possible winter variant of Lilina in a blue dress was Jossed with the reveal of the "Winter Dreamland" banner in December 2021, but that banner itself is a victim of another leak. (Though funnily enough, Lilina did eventually get a Bridal alt of her in a mostly blue dress with red accents.)
- A Chinese website leaked the "Winter Dreamland" banner in December 2021 more than half a day before the trailer was dropped, which revealed the winter variants of Mirabilis, Artur, Manuela, and Lysithea & Lute, with the latter two as a Harmonized Hero. The only one that was not revealed was the winter variant of Ignatz, who was confirmed in the trailer, but many players predicted that he was one of the two characters in the silhouette reveal the day before.
- The 34th Legendary Hero Banner in late-February 2022 had a two-fold leak. Data in Allegiance Battle in the "New Heroes & Ascended Idunn" banner update in mid-February 2022 revealed a Legendary variant of Caeda before a small update days later changed that to Regular Fjorm in order to prevent the game from crashing if one enters the Allegiance Battle mode. Later, the trailer for her banner was released around 3 days earlier than when it usually does, where trailers for banners outside of Special Hero banners normally drop a day before the banner goes live.
- When the FEH 5th Anniversary CD was found to contain 230 songs total in March 2022, it was discovered that all of them were already implemented in the game at the time. However, there was one exception: Leif's Army, Seeking Victory ~ Advance A from Thracia 776. This inadvertently revealed that the next New Heroes banner would be a Thracia 776 banner, which was proven true with the release of the "New Heroes & Ascended Mareeta" banner in mid-March 2022.
- A minor one, but the sprite for the Gronnvulture(+) tome was added alongside the "Unlikely Friends" banner in early April 2022, even though no unit in the banner uses it, hinting that there would be a Green Tome user in the next banner. Later that month, the "New Heroes & Ascended Ishtar" banner revealed that Arthur from Fire Emblem: Genealogy of the Holy War was the wielder of the tome.
- With the 6.7.0 update in early July 2022, alongside the Taken by the Tide units, the new Heroes original characters Ymir and Ganglöt were also found for the "Life & Death" Tempest Trials Series. Come the next update, the former was Promoted to Playable in the "Special Summon: Ymir and More" banner.
- With the 6.9.0 update in early September 2022, alongside the Bite of Flame units, the Rearmed version of Líf was also revealed as the boss of the "Life & Death 3" Tempest Trials+ instead of Harmonized Flame Tana & Peony.
- With the release of Regular Female Corrin's Resplendent and the announcement of Brave Roy's Resplendent in late-January 2023, the Resplendent webpage for Matthew, which was released in early-May 2022, briefly changed artwork and therefore leaked the Resplendent for Innes◊, who is wearing Askr attire.
- The 6th Feh Anniversary Feh Channel had three things leaked before their official announcement:
- In the 7.1.0 update in early January 2023, alongside the "Sage of Khadein?" units, dataminers also found that the Grand Hero Battles for Rutger, Regular Haar, Fallen Delthea, Cynthia, Kronya, Death Knight, Astram, Conrad, Cormag, Kempf, Travant, Heath, Flame Emperor, and Iago were going to be added to the daily Grand Hero Battle cycle on February 2nd, 2023, lining up with the 6th anniversary of Heroes. This was officially announced on the Feh Channel.
- With the Double Mythic Heroes: Fomortiis & Gotoh update in late-January 2023, an anonymous dataminer on Discord leaked the Valentine variants of Leo, Takumi, Hana, Duo Elise & Sakura and Effie for one of the Arena seasons, and while it was assumed to be fake due to not lining up with the actual datamined Arena script, the 6th Anniversary Feh Channel's announcement for the "A Special Gift" proved otherwise.
- Around a day before the 6th Anniversary Feh Channel's announcement, the CDN for the CYL7 website leaked assets showing the four winners of CYL7, those being Soren, Female Corrin, Male Robin, and Gullveig.
- After Male Robin's Legendary Hero banner released in late-February 2023, a leaker, who was presumed to be the same dataminer that leaked the "A Special Gift" banner the previous month, leaked the banner units for the 2023 Spring banner's Tempest Trials+ banner on Discord, which included Karla, Bernadetta and Minerva, with Minerva's presence hinting at Michalis due to Minerva already being on the 2021 Spring banner and Michalis's regular variant not being summonable through Orbs. Sure enough, the "Spring Eternal" banner in early-March 2023 revealed the Spring variants of Bernadetta, Triandra, Ashe, Michalis, and Karla & Freyja, whom the latter was also a Harmonized unit, and Michalis as a Tempest Trials+ reward unit.
- Regular Thrasir's Killing Intent+ from update 7.3.0 to update 7.4.0 in March-April 2023 had the unintended additional effects of granting her a guaranteed follow-up attack and ignoring any effects that inflict -X special buildup on her per attack during combat if there is a field stat debuff on any foes within 2 spaces of the target that triggered alongside the other effects of Killing Intent+. While it didn't fit her playstyle without skill inheritance, it was unusual that those two effects were connected together, hinting that there is someone who was supposed to have those effects, but very few heroes at the time could use those effects well without overlapping or contradicting their intended playstyle, meaning that it was part of a future unit's skillkit. Indeed, with the release of the "No Matter Where" banner in early-April 2023 that featured the Young variants of The Blazing Blade characters, those two effects were the last two listed on Young Duo Mark & Lyn's Total War Tome.
- In the "New Heroes & Mythic Heiðr" banner in early June 2023, dataminers found that Spurn 4 was included in the update, despite none of the units on the banner having it. This led to speculation that a future unit was going to have that skill. Indeed, it was later revealed that Summer Fjorm has it in her kit in the "Summer Firsts" banner in early July 2023.
- In the 7.7.0 update in early July 2023, dataminers discovered an update to the chibi sprite belonging to Freyr's normal version, suggesting that he may soon be released as a Mythic Hero. This was proven correct at the end of the month, as Regular Freyr was finally released as an Mythic Hero of Anima 3 years after his debut Book had ended.
- In the 7.10.0 update in early October 2023, dataminers found two things interesting: that the Peony that appears as a boss unit in the "Nihility & Dream 4" Tempest Trials+ was internally names "Peony2" instead of being named Ascended or Rearmed like previous heroes, similar to how Rearmed Líf in the "Life & Death 3" Tempest Trials+ was internally named "Lif2" before his official Hero type was named, hinting another new Hero type, and that the internal skill numbers for Sacred Seals, Duo/Harmonized skills and Refine effects were increased by one, hinting at another Skill slot between the Sacred Seals and C Skill slots, connecting to the new hero type that Peony was going to be introduced with. Indeed, the "Attuned: Peony & Triandra" banner trailer in mid-October 2023 revealed that Peony and Triandra in that banner were introduced as Attuned Heroes and that the new skill slot was for Attuned Skills.
