Mashima has never been one for planning ahead plot-wise. The Ass Pulls in both Fairy Tail and its sequel, Fairy Tail: 100 Years Quest, tend to revolve around never-seen-before magic developments (most of which are never foreshadowed, or if they are, it's in such a vague way it might as well not be) which occur without warning in battles or are demonstrated by different magic users.
Most are supported (read: handwaved) by a vague and nonspecific magic system that even in-universe is full of unknowns and is a subject of intense study (with subtle overtones of knowing too much leads to madness or These Are Things Man Was Not Meant to Know).
Warning: Due to the sheer amount Walking Spoiler characters that are involved in Fairy Tail: 100 Years Quest, all spoilers are unmarked.
Franchise-wise
- Natsu has been known to turn around battles in his favor through some form of Ass Pull - generally the Indy Ploy kind of Ass Pull where the character makes shit up on the spot. They are usually built up during the battle as Natsu struggles and fails with an obstacle before finally figuring out how to overcome it in an Ass Pull manner (routinely doing things that the other characters say are supposed to be impossible or extremely dangerous but often have no technical reason why they shouldn't work in the setting context).
Fairy Tail
- In order to defeat Zancrow, Natsu uses up all of his immediate reserve of dragon-fire flames and magic, making himself an empty vessel, which then allows him to eat god-fire flames without them conflicting with his own. Even Makarov is completely stunned that it somehow works, and yells at Natsu for trying something so reckless – making this an in-universe example at the very least. This evolution of Natsu’s ability to eat different flames and elements then culminates in the fight with Hades where Natsu takes in Laxus’s Dragon Slayer lightning and unlocks his Dual Dragon Slayer Mode.
- Erza's magic style lends itself to this trope rather easily. Due to using weapons and armor stored in Hammerspace for every situation she can think of, she's constantly pulling out new ones to suit her situation, some of which are never before seen. A particularly infamous example was the Nakagami armor, which was described as essentially making Erza invincible and allowed her to easily defeat Minerva despite the latter being far less injured. The title of this Chapter 321 review
sums up fan reaction nicely. - In her fight with Azuma, the "souls" of Erza's friends pushed her forward to pull off an OHKO sword swing against Azuma, an enemy that she had damaged and worn down but wasn't close to finishing off. This one can be explained by them being on Fairy Tail's sacred island and Azuma's attempted final attack drawing upon the magic of the very Tenrou Tree he uprooted (which also contains the magic of the Fairy Tail mages, which could have legitimately gotten away from him thanks to Erza's iron will and deeper connection), but it can still come off as contrived at first glance, with Makarov's later musings at the end of the arc on magic evolving alongside wizards only helping a bit.
- Erza's second fight with Kyouka, after Kyouka enters her One-Winged Angel form. Erza hit Kyouka's Berserk Button (telling her that she'll never get to meet Zeref), after which Kyouka started absolutely mopping the floor with Erza, culminating in Kyouka utilizing her Sensation Curse to rob Erza of her five senses. Erza somehow manages to pull through a win despite having none of her senses. This one is so much of an Ass Pull that even the characters are forced to admit that Erza won... because she's Erza, logic be damned.
- The red lightning that Laxus obtains to defeat Wall, who had been firmly established to No-Sell any lightning attack. It comes out of nowhere (at most, a flashback implies that it has something to do with Yuri), with no real explanation on how it works. At most, people have suggested from the name "Mercury Fulminate" that Laxus created the actual explosive compound, but the end of the chapter still attributes his victory to "lightning that surpasses lightning". This one has to wait until Fairy Tail: 100 Years Quest for a proper explanation in that it's Laxus' unique Super Mode as a Lightning Dragon Slayer, created by utilizing his own blood to empower his lightning.
- August's defeat. After Gildarts and Cana manage to land a single hit past his Power Nullifier, he decides to try and take out Fairy Tail with a massive spell that will kill him as well, which he ends up limiting to himself after seeing Mavis nearby and not wanting to hurt his mother. While his death is meant to be tragic, his decision to resort to a suicide attack makes little to no sense since Gildarts had lost the prosthetic arm needed to bypass August's defense, meaning the heroes had no chance of beating him, and August could have simply killed them at his leisure, making his death come across as incredibly contrived.
- In chapter 517, Wendy turns out to have survived Irene's Grand Theft Me by copying the same Enchantment magic Irene used on her. Even if it was established that Wendy and Irene use the same type of Enchantment magic and Wendy could conceivably copy it, the idea of Wendy successfully coping a body stealing spell she's never seen before, as it's being used on her, and casting it back in a fast enough time to body swap instead of having her mind destroyed is a little too convenient to call clever writing.
- Everything about Anna Heartfilia's role in the present counts as this. Neither Zeref nor Aquarius had ever once insinuated that Anna had come through as well during their independent accounts of the plan. Initially introduced as the person who simply opened the Eclipse Gate to enact the plan, we are later told that she was one of the Dragon Slayers' teachers alongside their parents, her absence from their own recollections explained away by their apparent affected memories from traveling through the Eclipse Gate. She has flimsy reasons for never contacting the Dragon Slayers or Zeref, and gave little information about where she'd been while apparently knowing that her descendant Layla died and made no effort to comfort Lucy at the time. Her main role was to introduce the space between time, a tiny black hole (explicitly compared to the size of a tangerine) floating over the open ocean that she somehow found and then prepared to try trapping Acnologia in it.
- Doranbolt was a Fairy Tail spy from the beginning came out of nowhere since no one except Makarov knew about it. Even Doranbolt didn't know, due to sealing away his own memories. It's established that Makarov visits Doranbolt periodically and undoes the memory spell he placed on himself, but Doranbolt still lampshades how weird and confusing it all is. Word of God admits that it was a plot point that had been considered a long time ago and was nearly scrapped, thus leading to the lack of foreshadowing.
- The Alvarez Empire has no presence in the story up until the arc focused on it despite supposedly being a looming threat toward Ishgal throughout the series. Then we learn who its Emperor is: Zeref, who had been portrayed as The Aloner because of his curse killing all life around him as long as he values it. He claims that he's able to avoid killing his empire's citizens by not viewing them as people, yet he's able to maintain a good enough relationship with them that they still adore him, making his previous issues with killing those around him seem self-inflicted. It also calls into question what he was doing on Sirius Island when he has an empire to rule, even if it is implied he apparently spends long periods of time away from the empire to leave day-to-day activities to his ministers and the more rational Spriggan 12. The revelation seems to have been thrown in just to put Zeref directly into the position of Big Bad.
- The only thing surprising about Irene's relation to Erza was how long the story delayed such an obvious reveal. Then we're told that she's the Queen of Dragons and the creator of Dragon Slayer Magic, despite there being nothing obvious about Irene beforehand having hinted at any relation to the Dragons. The Dragon she learned from, Belserion, is suddenly revealed to have started the Dragon King Festival to protect humanity despite Zirconis' story heavily implying that it was Igneel. The revelation, as well as the consequences of her dragonfication, comes several chapters after Acnologia is shown to be capable of casually switching back and forth (granted, it's not shown how he first transformed back or even his own initial reaction to his transformation), and even the current Irene shows no sign the Sense Loss Sadness that she's supposedly suffering from. On the topic of Acnologia, he displayed the ability to smell all the Dragon Slayers across the continent, yet had absolutely no interest in Irene until he was goaded into a fight. When brought up, Hiro claims that he always planned for Irene to create Dragon Slayer Magic, but hadn't decided on her relationship with Erza, and had to leave out any aspects of her backstory before then.
- Fairy Tail: Dragon Cry: The Quartum Army. They're not mentioned beyond a possible nebulous "dark magic" that Zash uses, and appear just in time to make the fight more dangerous for Fairy Tail. The twist? They're fourth-generation dragon slayers.
Fairy Tail: 100 Years Quest
- The basic concept of the titular quest falls into this after the original series had repeatedly stated that the age of dragons ended with Acnologia, suddenly introducing not only five dragons on his level, but an entire guild of Dragon Slayers who sustain themselves on dragons, all of which Acnologia inexplicably missed on his four-century rampage across the world. The Dragon Gods at least get the excuse they were purposely hiding from Acnologia for the past 400 years trying to get strong enough to fight him, but with the amount of dragons outside of them that Acnologia also missed causes people to question how in the hell Acnologia managed to miss so many.
- This was only made worse when Kirin's backstory was revealed, stating that Georg killed his dragon, Elexion, only 30 years before the main story, when Kirin was still a kid. Meaning that the various dragons Diabolos was responsible for slaying and consuming were all walking around and using their magic power relatively recently, and yet Acnologia still missed them somehow. Somewhat mitigated with the reveal of Athena, the true White Mage, whose powers are on par with Zeref's, the Black Mage. Zeref was the only known being in Earthland with the power to put Acnologia in check and allowed him to do as he pleased, and if Athena could match that threat level but did not give Acnologia free roam over Guiltina, it would explain why Acnologia kept his focus limited to Ishgal.
- The "White Mage" allegedly being just as big a threat as Zeref, whose curse and creations were the backbone of much of the original story, despite her being a newly introduced character in the sequel. This died down a bit with the revelation this "White Mage" is an imposter just using the name of the real one, who went missing. When the actual White Mage, Athena, appeared, her absence was explained. She belongs to the master of Gold Owl, an alchemist guild trying to bolster her power as a living doll created through alchemy.
- The Soul Dragon Slayer Wraith's true identity. Wraith is a spirit suffering Ghost Amnesia, and had joined Diabolos due to their being able to see him and wanting to find out who killed him. Right as it looked like he had Natsu on the ropes by possessing Makarov, he suddenly remembers who he was, specifically because of Makarov's memories: a previously unmentioned friend of Makarov's and Fairy Tail member that was killed in a random monster attack years before the series started, remembering such allowing him to finally find peace and pass on after surrendering to Natsu. Not only does the revelation come out of nowhere, it neither explains how Wraith became a Dragon Slayer, let alone a 5th generation,* nor why only Dragon Slayers can see him, and reads like a Writer Cop Out to justify Natsu not having to seriously hurt Makarov in a fight.
- Erza magically taking control of Misaki’s Blue Dimension away from her and turning it Red, which allows her next strike to utterly floor her opponent. Keep in mind that Misaki is stated to be comparable to Gildarts in terms of power, has had control over this dimension for an unspecified amount of time whereas Erza was literally just introduced to it, and yet she immediately proves to be Misaki’s better.
