The deus ex machina (or "God in the machine") gets a lot of flak from shonen anime fans, even though most of them don't fully understand it. Deus ex machina aren't inherently bad because they can come out of nowhere or help the heroes suddenly win. My Hero Academia and One Piece show how the trope can be used well.
A Deus ex machina becomes bad or glaring when it helps the heroes win in convenient yet unconvincing ways. These Deus ex machina were deployed in the most frustrating ways imaginable. Worse, these Deus ex machina diminished their heroes' efforts while also watering down their anime's tension.
Spoilers ahead.
10 Chao Lingshen & Hakase Satomi Had A Time Machine
Negima!
Negima! was faithful to its manga for most of its run, but it suddenly changed things in the final act. Asuna Kagurazaka died on her 14th birthday, and it seemed as if Negima! would end by dealing with the tragedy's aftermath. Luckily for Negi Springfield and friends, Chao and Hakase had a time-traveling watch that could undo everything.
The Cassiopeia watch existed in the Negima! manga, but it was a recurring item that was also better explained. Since the anime cut or compressed a lot of material, Cassiopeia was abruptly introduced as a last-minute deus ex machina. This lazy deus ex machina nullified whatever emotional gravitas Asuna's death could've had.
9 Vylk Had Royal Blood This Whole Time
The Promised Neverland
One of the biggest pitfalls of The Promised Neverland's second season was its laziness. Complex conspiracies and clashing ideologies were hurriedly resolved by deus ex machina. One of the most egregious was Vylk, the demon who Emma befriended. Vylk seemed to be an ordinary demon, only for him to have royal blood.
The royal blood was one of The Promised Neverland's key plot points since it cured demons of their hunger. In the manga, Mujika was the only demon who had the blood. The anime-only Vylk conveniently had it as well, meaning there was lots of blood to share. That, and Vylk led the demon uprising off-screen.
8 Conan Edogawa Always Solved The Case
Case Closed
The best and worst thing about Case Closed is its strict adherence to formula. No matter how unsolvable the case was or how obscure the evidence was, Conan (or Shinichi Kudo) would find the culprit. This could be written off as Conan being a master detective, but his convenient eureka moments quickly lose their novelty.
Case Closed's deus ex machina came in the form of telltale evidence that Conan only noticed at the last second, the culprits succumbing to guilt just before the episode ended, and everyone being too distracted to notice that Detective Kogori Mouri fell asleep. Conan's few close calls were the only thing that broke up the monotony.
7 Alluka Zoldyck & Nanika Were A Living Deus Ex Machina
Hunter X Hunter (2011)
To most Hunter X Hunter fans, Alluka (Killua's sister) and Nanika (the entity living inside her) are more deus ex machina than characters. What's more, Nanika's Nen ability is a literal deus ex machina. Nanika essentially has the power of equivalent exchange. She can wish for anything, but for an equivalent price.
When Gon fell into an unresponsive state after fighting Neferpitou, Killua ordered Nanika to revive him. Not only did Nanika bring back Gon in perfect health, but she did so without anything of value being taken away from Killua in exchange. What made this frustrating was how the fight's high stakes and Gon's hubris were so easily undone.
6 The Heart Of The Cards & Friendship Decided Duels
Yu-Gi-Oh! Duel Monsters
Yami and Yugi's "Heart of the Cards" is Yu-Gi-Oh! Duel Monster's most (in)famous deus ex machina, but he wasn't the only one who used it. Joey, Tristan, Téa, and all of Yami and Yugi's friends didn't win with clever tactics. Instead, they used the power of friendship to convince their decks to give them the right (overpowered) card.
Yu-Gi-Oh! Duel Monsters also didn't follow the real game's rules or any kind of logic. Cards operated however the plot demanded, making them deus ex machina as well. For example, Maiku, the Magical Mist's effect changed to whatever Yami currently needed. Similarly, Yami beat Arkana after the latter's Dark Magician card defected.
5 Fairy Tail Turned Friendship Into A Literal Deus Ex Machina
Fairy Tail
Fairy Tail is polarizing among anime fans because of how diligently it obeys shonen clichés. Nowhere was this made clearer than in how Fairy Tail weaponized the power of friendship. This is somewhat justified by the magic system being dependent on the mages' emotions, but it got ridiculous at times.
Natsu and friends almost always won after an idyllic flashback empowered them. Fairy Tail's enemies being individualists who scoffed at having friends also helped them. The power of friendship has always been one of shonen's favorite deus ex machina, but Fairy Tail overused it so much that it felt like self-parody to some viewers.
4 "Plotkai" Became Infamous For A Reason
Bleach
A lot has been said about how the very human Ichigo Kurosaki is overpowered and blessed with every supernatural race's powers. These unfair odds also applied to the other Shinigami. No matter how powerful or godlike their opponents were, the Shinigami always won thanks to a last-minute deus ex machina that fans dubbed "Plotkai."
Plotkai (derived from "plot" and "Bankai") refers to abilities and power-ups that were never foreshadowed and only used for one specific fight. By merely existing, the Plotkai denied Bleach of any stakes. Examples include Zaraki Kenpachi's one-time mastery of basic kendo and literally all of Mayuri Kurotsuchi's inventions.
3 "Plot No Jutsu" Was Coined By Frustrated Fans
Naruto
Naruto is an important anime for many fans, but its nostalgia was tainted by an abundance of deus ex machina. These contrivances were so common that fans mockingly named them "Plot no Jutsu." Whenever Naruto or anyone else miraculously survived or used a convenient technique, it was dubbed Plot no Jutsu.
The morally complicated Sasuke was the guiltiest user of Plot no Jutsu. Sasuke won battles that should've killed him by using a Plot no Jutsu, or because someone gave it to him. The most egregious examples are Sasuke summoning the snake Manda to survive Deidara's explosion and the Sage of the Six Paths giving him godlike powers to defeat Madara.
2 The Z Fighters Got All The Deus Ex Machina
Dragon Ball Z
Dragon Ball Z became legendary thanks to its fights, but not all of these were won fairly. Goku and the Z Fighters often won only because of a last-minute save. These plot contrivances either came in the form of convenient power-ups or literal deus ex machina. As a result, Dragon Ball Z's stakes were constantly nullified.
For example, the Hyperbolic Time Chamber and Zenkai Boost were used sporadically despite their lack of limitations. Additionally, Vegito defused in the world-ending Super Buu even after his fusion was said to be permanent, and Porunga's limit of one resurrection at a time was conveniently undone just before Vegeta wished Earth back to life.
1 Everything Went Kirito's Way
Sword Art Online
Kirito's very existence is one of the reasons why Sword Art Online is so divisive. Kirito is often derided for being a transparent audience self-insert, and his excessive deus ex machina embolden this. Regardless of how dire the circumstances were or how powerful his opponents were, Kirito always won through plot contrivances.
Kirito's deus ex machina broke all internal logic and stakes during the Aincrad arc. For example, he resurrected Yui by easily hacking the game's source code, and he kept himself alive through sheer force of will even after Kayaba reduced his HP to zero. Sword Art Online's use of deus ex machina only got more egregious after this.