Ascended Fanon: The 3D maneuvering soldier with afterimage effects in the opening actually wasn't supposed to be Jean, but rather a soldier representing humanity. However, given the number of people claiming it to be Jean, the production staff has acknowledged it as so.
Banned in China: The series was denied a Chinese release for excessive violence and juvenile delinquency.
Beam Me Up, Scotty!: The first line of the first OP Guren no Yumiya has been misspelled in numerous ways due to Linked Horizon's inexact pronunciation of German and inconsistent fansubs, one common one being "Sie sind das Essen und wir sind die Jäger!" The official spelling is "Seid ihr das Essen? Nein, wir sind der Jäger!".
In episode 4, Hannes' uniform has the Survey Corps symbol on it.
In the manga, the same mistake is done with Annie's uniform having the Survey Corps symbol during the scenes where Eren and co. attempt to trap her.
Jokes and theories about Erwin being a Shifter have cropped up after Chapter 57 when he placed his right arm on Reeves' corpse, despite it having been bitten off by a Titan a few chapters earlier.
In Chapter 71, Instructor Shadis's beard inexplicably disappears during a close-up on his face.
in Chapter 112, the room where Armin, Mikasa, and Gabi are in has a double-door that magically turns into a single-door when Eren walks through it, and then turns back into a double-door the next time it's shown.
A few egregious ones occur in Chapter 119.
A few chapters earlier, it was shown that the crater in Shiganshina District left by Bertolt's explosive transformation was turned into a makeshift lake. When an aerial shot of Shiganshina District is shown when Zeke turns all of the infected soldiers into Titans, the lake is nowhere to be found.
Another is when Eren breaks out of his Titan wearing both pants and shoes despite having both legs bitten off by Porco before transforming.
After the production staff liked the idea of Jean being the backflipping soldier in the opening (see Ascended Fanon), Jean was shown performing that exact move as an escape maneuver when his group encountered the Female Titan.
Chapter 110 gives us two about fan favourite characters:
Zeke's ironic statement on Levi's popularity:
Zeke: I bet you're not popular with the ladies. [...]
Levi: [...] I've been popular before...
Hitch remarking on a sleeping crystallized Annie:
Hitch: How are you this popular when all you do is sleep?
Less prominent, but Ymir is consequently referred to as Freckled Satan. Ironically, there are also people who have referred to her as Freckled Goddess.
"Gesumin" for Armin, when he's being a Manipulative Bastard. ("Gesu" is Japanese for "scumbag".) Coined by Isayama himself, when he posted the original version of the scene where he sports a Slasher Smile while lying to Bertolt about Annie being tortured, showing just how disturbing the original expression was. It was captioned, "This Gesumin... What should I do with it?".
"Geographia" for Frieda Reiss, who was shown in Eren's flashback in chapter 53, due to her Uncanny Family Resemblance to her half-sister, Historia Reiss.
Some jokingly call Historia's mother "Rosa" due to physical ressemblance and her wishing her daughter was never born.
Abe Lincoln, John Marston and Clint Eastwood for Captain Ackerman due to his physical resemblance to them. Also "Kanye", due to early scanlations translating his name as Kaney coupled with his tendency to start lines with "Yo [Insert Name Here]".
Following Darius Zackly's confession that his overthrowing of the puppet monarchy was essentially for shits and giggles, fans quickly christened him as "Dickly".
Kenny Ackerman's second in command is called Not-Annie due to very similar look and because she hasn't been named yet.
Due to how it looks in certain shots, Rod Reiss's Titan form earned the name "Roast Chicken Titan", or "Giant Turkey Titan". Upon its appearance in the anime, it was also named the "Maggot Titan".
The Survey Corps soldier who made it out of the rush on the Beast Titan unscathed and carried a wounded Erwin away was often called Not-Jean due to his perceived resemblance. His official name was revealed to be Floch a few chapters later.
The PS4\PC game only has the Wings of Freedom subtitle in Europe. That said, some non-Europeans refer to it with the subtitle to distance it from the crappy 3DS game.
Titans in general have commonly been referred to as 'meat Gundams' after it turned out that they were really pilotable Instant Armor, creating a significant early Genre Shift for the story.
Many fans suspected the Female Titan to be Annie Leonhart long before the revelation came due to the way it resembled her.
Likewise to the above many people guessed that Reiner was the Armored Titan and Bertolt was the Colossal Titan long before it was officially confirmed for many of the same reasons as the Female Titan revelation.
Many people also predicted Titans being transformed humans.
Most people guessed that Marcel was a Titan shifter and eating him allowed Ymir to become one too long before it was officially revealed, and the repeated instances where his demise is shown in the manga (at least 3 times) have led to the fandom adopting it as a running gag, stating that they'll finally see his death.
It's confirmed that Eren ate his father, which many people have theorized for a good while.
Many speculated that Annie was involved in Marco's death before it was finally confirmed.
Fans were quick to point out that Zeke and Grisha looked practically identical, leading many to suspect that they were related, or at the very least knew one another personally.
Many have correctly predicted that humans weren't actually extinct beyond the Walls.
The identity of the Smiling Titan as Dina Yeager, Grisha's first wife, was pretty much speculated since she was introduced in Chapter 86 and was confirmed in the next chapter.
A few people speculated that the Shell-Shocked Veteran that Falco stops to help in Chapter 94 was Eren in disguise, come Chapter 97 and they were proven right.
Killer App: The anime series was one of the reasons to sign up for Crunchyroll.
The recap episode of the first thirteen episodes of the first season was never dubbed in English by Funimation.
As are the OVAs, but on the other hand, they're included in certain special english editions.
The series wasn't released in China due to its juvenile delinquency and graphic violence.
Old Shame: According to an interview that comes in the Blu-Ray box set of Season 2, the "Uprising" arc (manga volumes 13-16) was where Isayama felt "the most disappointed with his work" and hopes that on its animated adaptation in Season 3 he'll have the chance to wipe away that regret and replace it with something he's proud of. In context, the adapted Uprising arc changed in terms of pacing by cutting off some characterization from Dimo Reeves' negotiations with the Survey Corps and his importance in the district of Trost, and the journalists Roy and Beaure's impact on the reporting of the crimes of the government.
Many members of the dub cast, like Patrick Seitz and Josh Grelle, watched the simulcast of the Japanese version. The most surprising example of this, however, is the dub VA for Eren Jaegar: since FUNimation was announced to be the English distributor of the series, no one could've expected Bryce Papenbrook, a California-based VA and huge fan of the series, to be announced as the leading man of the anime.
Director Tetsuro Araki himself is also a fan of the manga and was very excited when he got to direct the anime.
Yoshiki Hayashi of X Japan happens to be a fan as well. Then, it's announced that he, together with HYDE would perform the opening theme for the third season entitled "Red Swan".
The Novelization of Levi's backstory, A Choice with No Regrets being adapted into a Manga, then being adapted into a two-part OVA.
The soundtrack composer for the anime was heavily influenced by the music of Dutch Symphonic Metal band Epica. In December 2017, Epica released a cover album of songs from the anime in Japan (worldwide release July 2018).
Schedule Slip: The creators of the anime were caught completely off guard by how popular it was, resulting in a three year gap before they were able to make a second season.
Screwed by the Network: A variation where the show itself indirectly resulted in this for something else. When the Attack on Titan anime first started airing on Toonami (which at the time aired from 11:30 PM to 6:00 AM), it was placed at the prestigious 11:30 PM timeslot and preceded to bring in some of the best ratings for the block. The higher ups at Turner, seeing the show's ratings potential, told the production crew for Toonami to keep the show at 11:30 PM when it went into reruns. The result was lower ratings for the block overall, and it got bad to the point where [adult swim], in a knee-jerk reaction and an effort to counter the low ratings, decided to shrink the block down by 3 hours, resulting in Toonami now airing from 12:00 AM to 3:30 AM and most of the shows airing on the block going off the air.
Shrug of God: Isayama does this quite a bit, in combination with his trolling of the fandom. Among the things he has shrugged and left up to the fans or otherwise refused to clarify:
Hange's gender is whatever fans want it to be. (He also invited fans to decide any character's gender, including those with a stated one.)
Rumored to have stated that any and all pairings may be canon if fans want them to be.
Sleeper Hit: Since its debut, it has become one of the best selling shonen manga in Japan and positively exploding with the anime. In 2013 volume sales only the Pirate King survived the Titan onslaught as people furiously bought up the back issues.
Star-Making Role: For Bryn Apprill as Krista. This was the first role that got the attention of the English-speaking fanbase.
Hajime Isayama enjoys messing with the audience, which includes releasing fake◊ volume◊ previews◊ and refusing to clarify Zoë's gender solely because it confused people.
Not exactly from Hajime Isayama, but there was an announce that dakimakura of Shingeki no Kyojin would be released. Of course anyone would expect to see Eren, Mikasa, Armin, Levi, Annie and the like, but when one was released...◊
And Isayama strikes again. He was asked to promote a new series so he drew an advertisement band for the first volume. This◊ was the result◊.
With the release of the second Guidebook, "Outside", the cast now have birthdays. Reiner's birthday is stated to be August 1st, which is celebrated as "Yaoi Day" by fans of the genre.
It gets better: Marco's birthday is revealed to be June 16th. That's exactly half of the year.
Isayama strikes again! When a fan at a Q&A event asked what type of woman Levi preferred, Isayama responded by questioning whether Levi liked women and suggested he just likes tall people. Which covers every single popular ship.
Troubled Production: While this series was what cemented WIT Studio as a major anime studio, the creation of it has been extremely taxing on the staff. The TV broadcast of season 1 was on an infamously short-staffed breakneck schedule and featured tons of cost-cuttingmeasures, particularly during the Battle of Trost arc. The final episode of season 2 needed a whopping 25 animation directors to pull it off. Season 3 had a completely unexpected six month break after Episode 12 aired, to the shock of the fanbase (except for the people who paid attention to Japanese TV schedules and realized it was coming a few weeks beforehand), and the second half of the season was only 10 episodes long when it had already been taken for granted that it would be a 24-episode season.
With the above in mind, rumors started circulating wildly in mid-May 2019 that WIT Studio was leaving the franchise after the conclusion of season 3. The genesis for these rumors was a public comment by a Chinese contracted animator on the social media giant Weibo where he made an unclear comment that this season would be the last for the studio, and then going radio-silent afterward (his wording was ambiguous enough that even Chinese readers flooded his account demanding clarification, to no avail). The rumors picked up further steam when Western anime leakers and anime industry watchers also started dropping hints that the same thing was happening (but again, furnishing no concrete proof). Finally, director Tetsuro Araki was outright asked at a concert shortly before the season's end if there would be a season 4: he confirmed an S4 was happening (which was proven by a teaser trailer at the conclusion of S3) but did not answer any questions about what studio would be doing it.
Hajime Isayama first brought his pilot to Shueisha, with the hopes of having it published in Weekly Shonen Jump, but the editor who evaluated his work said that it was good, but not good enough for Jump standardsnote "This is manga, we want JUMP!" being his exact words; then, when Isayama gave his pilot to Kodansha, it was accepted in no time and published at Bessatsu Shounen Magazine, a monthly offshoot of their Weekly Shonen Magazine. The first two volumes sold more than a million copies combined. As a result, Jump, and consequently Shueisha, lost a giant new hit to its biggest rival.
The pilot itself, now called Attack on Titan: Volume 0, is extremely different from the series that we know today. For starters, third-dimensional maneuver gear don't exist in the pilot, and humans are fully capable of "jumping 10 meters" while fighting Titans.
When Annie is called out as the Female Titan in chapter 31, Isayama originally drafted her reaction to be along the lines of a (disturbing) Tearful Smile, as a result of being "relieved" she'd been found out. He regretted his decision of changing it and requested that a similar reaction would be added to the anime.
Isayama had originally planned to kill off Sasha in chapter 36, but later abandoned that idea. It was apparently so brutal that the editor went to the bathroom and cried.
In the same interview describing the initial plot of chapter 36, Isayama stated he originally had Eren knowing all along that he could turn into a Titan, but later dropped the idea. Overall, the manga has leaned on hinting that Eren was meant to inherit the Attack Titan, and that both said predestination and the eventual attainment of the Titan Power start affecting his decision-making
In an interview, Isayama revealed that Pieck was originally designed as a middle-aged man◊, but ended up as a young woman instead.
The guidebook released in 2017 revealed that Armin and Jean were originally a single character in the earliest version, and that the character was split in two to create the roles for those two characters.
Word of Gay: The official website confirms that Ymir's feelings for Krista are romantic in nature. Since then, Ymir's love for Krista was a plot point.
Word of Saint Paul: Anime Producer George Wada stated at the Animagic 2014 series panel that Krista and Ymir are a couple.
The walled city is inspired by the walled city of Nordlingen in Germany. The walls were inspired by Isayama's hometown, which was surrounded by mountains. Isayama wanted to go beyond the mountains, so he transferred his desire to Eren, who desires to go outside the walls and see the outside world.
Isayama revealed that the idea for Titans themselves came from encounters he had with drunk people while working at an Internet Cafè. The difficulty to communicate despite being the same species inspired him, and it made him consider that the most familiar and scary animals in the world are humans.
Some aspects of Erwin's personality were inspired by Ozymandias, but the primary inspiration for his character likely was Erwin Rommel. The fictional Erwin's birthday is on Rommel's date of death. Rommel was opposed to the mistreatment of Germany's Jewish population and attempted to assassinate Hitler.
Similarly, some of Levi's personality was inspired by Rorschach.
Mikasa is referred to as being worth a thousand soldiers, which is meant to be a Shout-Out to Tomoe Gozen, a famous female Japanese warrior whom legend said was worth a thousand men on the field of battle.