Koro-Sensei in Assassination Classroom promises to destroy a tentacle reducing his speed for every top spot the class gets on their finals. The delinquents capitalize on this by studying and acing... Home-Ec?! It may not be one of the major subjects, but it's still one of the finals.
Scar from Fullmetal Alchemist is from Ishbal, and the Ishbalan people despise alchemy as a defamation of God's natural order. When Scar makes it his mission to use his alchemic right arm to take revenge upon the State Alchemists who committed atrocities against his people, he tells himself that he's not actually partaking in the "sin" of alchemy, because true alchemy is three stages (Composition, Deconstruction, and Reconstruction), and he only stops at the second, which is still enough to blow a person's body apart.
The "games" in Death Parade essentially run on this, though it's never explicitly stated. The rules told by the arbiter of the game are literally the only rules, meaning that players can assault one another, cheat, or trick each other as much as they want, so long as it wasn't explicitly stated otherwise. And to top it off, the rules that are stated are pretty much tailored to bring out the worst in each player, so they're essentially made up on the spot.
In episode 8 of Code Geass R2 (second season), Lelouch (as Zero) makes a back-room deal with Britannia where he bargains for Zero to be exiled from Area 11 (AKA Japan) rather than executed for his terrorism. During the meeting, when asked if he's the original Zero, Lelouch gets the Britannians to admit that the face behind the mask doesn't matter; if someone wears the mask and shares his ideals, they're Zero for all intents and purposes. Then at the event where the exile is announced, Lelouch's Japanese allies don Zero costumes. Since the alternative would be slaughtering them to a man (not a very productive PR move), the Britannians are forced to exile one million Zeroes, who gladly join Lelouch's cause.
The rules for Martial Arts Rhythmic Gymnasticsclearly state that the only valid method of attack is with a tool. Barehanded (or barefooted) contact is strictly forbidden and can result in immediate disqualification. However, this doesn't stop Kodachi from modifying her gymnastics tools into clubs with retractable spikes, ropes which snap into iron rods, razor-sharp hoops, and explosive balls... or simply snagging whatever she can wrap her ribbon around (including Kuno) and hurling it back at her enemy. It doesn't keep Ranma from using anything she can get her hands or feet on (including Akane's pet piglet,) or demolishing the ring itself with kicks and punches. Kodachi using her bokken-wielding brother was also considered a valid attack.
It goes one step further - the rule forbids attacking your opponent without a legal weapon. Ranma wins by kicking the post Kodachi's standing on, breaking it and dropping her to the floor. Ranma then clings like a limpet to the broken halves of the post to stay off the floor herself.
In both versions this is how Kodachi "justifies" remaining the first true member of Ranma's Unwanted Harem (not counting Akane Tendo, on virtue of her being the Official Couple), despite having promised to give up on Ranma if she lost the match. Having agreed to "abandon her present affection for Ranma", she explains that she now "burns with entirely brand new passion for him" when she tells girl-type Ranma that she will be coming after Ranma again.
In anime version of the climax to the Phoenix Pill saga, Ranma needs to win a combination of downhill race and running battle in order to win the pill that will undo the Full-Body Cat's Tongue point that keeps him locked in female form. Unfortunately, he's never skied before, so he stinks at it. Akane mercifully throws him some snowshoes and points out there's no rule saying he has to ski down the slope, just that he has to reach the finish line first with an unbroken miniature snowman. Ranma takes things a step further by soon after hitching a ride on the back of an angry black bear.
In Mamotte! Lollipop, female lead Nina is distressed to learn that the contest for gaining a professional wizard's license has no rule against killing human beings (i.e. her) to obtain the Magic Pearl.
Revolutionary Girl Utena: Ain't no rule that says a girl can't wear a boy's uniform! Apparently there's no rule that says it has to be the same color as everyone else's, either!
The manga's Greatest Under the Heavens martial art tournament's regulations are full of loop holes. If you don't touch the floor out of the ring, you can fly, grow, become a giant monkey, multiply yourself, use fire-guns, stink and a long etc.
They do have one moment of Obvious Rule Patch: Goku uses the Nimbus (a controllable semi-solid cloud) to return to the ring after being launched out of it (this was before he could fly). The officials find themselves in a dilemma: Goku didn't technically touch the ground outside the ring, but do clouds count as "out of the ring" if one can stand on them? They finally allow it this once, but also state that if Goku does that again, he'll be disqualified.
Similarly, in Dragon Ball Z's Cell Games, Cell decides to eliminate the possibility of either side losing by a simple ring-out by destroying the whole arena.
This is used hilariously yet again at the Other World Tournament where an epic, multi-episode long battle between Pikon and Goku was rendered a draw when both contestants were disqualified. Why? Both had touched the roof of the dome (the dome is the size of a small galaxy) and technically that was a ring out. Even more hilarious was that both had touched it at the same time, so there was no winner. This comes as a surprise because the entire scene appeared to be just another filler action scene until the Grand Kai pointed it out.
The real punchline comes from the fact that the abuser is the Grand Kai himself. His shaky justification is that "If you stand on your head the ceiling is the floor" and gives them a consolation prize of still getting to train with him (which was the offered prize for the tournament winner), but after a century. The real reason he does all this is because he realized during their fight that they both are even more powerful than himself, using the fake loophole as a stall so he can get back in shape in time to train them.
The Flying Nimbus won't let anyone ride it but the pure of heart, which early on in the series pretty much just meant Goku. Krillin and Bulma were both able to get around this by hanging off Goku's back and using a device to shrink small enough for Goku to carry in his shirt, respectively.
King Kai is able to find one during the Frieza saga with the Dragonballs. Porunga, the Namekian Dragon, can only be active while Guru is alive. But, Guru passed on due to the strain of his people dying. With Kami still alive, King Kai quickly poses a question: could people indirectly killed by someone - such as heartbreak - be revived? Kami realizes that it's a major possibility and they try it. It works.
Dragon Ball Z Abridged takes it one step further as this wish revives Nappa. Justification? Vegeta, at the time, was still one of Frieza's men, thus Frieza was indirectly responsible for Nappa's death.
In the Dragon Ball Z: Battle of Gods, Beerus says that he will destroy the Earth if no one is able to defeat him in combat. Despite pleas to the contrary, he states that he always follows his word. When Goku ultimately fails to defeat him, but still gave him the best fight he'd had in millenia, he destroyed a nearby rock and claimed that, oh dear, he seems to be too worn out to blow up the planet right now. He'll come back in a couple million years and finish the job. He never said he'd destroy Earth immediately, did he?
The whole legend behind becoming a Super Saiyan can possibly this this as well. At the time, the legends stated that only a saiyan pure of heart could become one, a perfect fit for Goku. At least until Vegeta learns how to do it. Ain't nobody said you couldn't be pure evil.
In Dragon Ball Z: Resurrection ‘F’, turns out there's a loophole in resurrecting someone after they died: Shenron can't revive them whole if they've been dead for over a year, but he'll gladly revive their spirit and the state their body was in when they died. Thus, when Frieza is resurrected, it's still in little pieces.
The whole point of ninja training in Naruto; one is supposed to barrel on through the rules.
The written part of the Chuunin Exam is one example. The test consists of ten incredibly difficult questions and cheaters are disqualified after being caught cheating a few times. Similarly to the real life Spartan example below, the true purpose of the test is not to determine the candidate’s knowledge, but to test their ability to gather the required information without being caught cheating. Naruto accidentally discovers a real loophole in the written test: you only fail if you’re caught trying to cheat, so there is no penalty for not trying to answer at all. He therefore passes the test by turning in a blank sheet of paper (and correctly answering the last question, which was a Secret Test of Character). Sakura also passes without cheating- she's smart enough to just answer the questions the regular way.
Naruto also does this with training which does not involve rules— any time he can't get past a certain stage of a training he always finds some clever way to bypass it. For instance, Jiraiya figured he couldn't get past a point in the Rasengan training because he couldn't randomize the rotation of chakra with one hand—so Naruto just used his other hand to make the rotation random himself.
His shadow clones are Loophole Abuse incarnate. Any time he needs to do something that can't be done (either at all or in X amount of time), shadow clones help circumvent the rules. It's how he managed to master the Rasengan (use a clone to focus the chakra) and then later evolve it into the Rasenshuriken (a second clone injects Wind-elemental chakra).
Liar Game. The day Akiyama actually plays a game without looking for loopholes will be the day of the apocalypse.
Deducing the hidden meanings and ramifications of the rules, then exploiting them to win is the wholly intentional part of the eponymous game.
During Eclipse's song-length game in Basquash!, the rules are that the game shall run until the end of the song. Rouge is "hurt", trying to force defeat on Dan's team, but Flora exploits a loophole when she reveals that Eclipse don't have to be the ones singing. She leads the crowd in song, letting the game finish.
After defeating Giovanni, the Viridian Gym Leader and leader of Team Rocket, Red refuses to accept the Earth Badge, on the grounds that he won't accept anything from Team Rocket's boss. Giovanni's response? Disband Team Rocket, so Red can accept the Badge from him as a Gym Leader and not a Rocket.
When Yamamoto orders Kenpachi and Byakuya to take Hitsugaya's team back to Soul Society to prevent them from helping Ichigo rescue Orihime from Hueco Mundo, Byakuya uses Yamamoto's Exact Words to exploit a loophole: Yamamoto only ordered him to bring the team back to Soul Society and nothing more. As a result, Byakuya brings the team back to Soul Society then promptly helps Renji and Rukia to return to the World of the Living where Urahara sends them to Hueco Mundo after Ichigo's group.
In the Amagai Arc, the captains are ordered to capture Ichigo for breaking Soul Society law. Byakuya turns control of the sixth division over to Renji because he does not have the freedom to break the rules whereas Renji has greater flexibility. That way, Byakuya can turn a blind eye to anything his crazy lieutenant does without breaking the orders he's under to not help Ichigo.
Uryuu thinks he's pulled this on his father to help Ichigo rescue Orihime from Hueco Mundo. Ryuuken restored Uryuu's powers in return for the promise that Uryuu would never again associate with Shinigami or their allies. Since Ichigo's been abandoned by Soul Society and is only a Substitute, not a real Shinigami, Uryuu concludes he's not breaking the promise to help. Ichigo calls it "twisted logic". Although the kids don't realise it, the manga implies Ryuuken planned the loophole and expected Uryuu to exploit it. The final arc reveals Uryuu's logic was sounder than the kids knew; unlike the kids (including Ichigo), Ryuuken knew that Ichigo's half-Quincy.
The Davy Back Fight of One Piece allows the team that wins each match to pick a member of the opposing team and force him or her to join their crew. In the third round, Nami considers choosing Foxy, who is set to compete in the Combat event against Luffy, so that they win by default in the third match and can easily get Chopper back. While some members of the opposing team hypocritically protest (they had already done the same thing to one of the three Straw Hat set to compete in the second event), Robin notes that this is legal, but none of the Straw Hats want Foxy with them.
After an Obvious Rule Patch and a boxing match, 497 Foxy members are forced to join Luffy's companions. Luffy's ship, the Going Merry, cannot carry that many people, besides he doesn't want them anyway. However, a pirate captain's orders are absolute, and there's no rule a captain cannot assign his crew a mission to be dismissed.
Actually inverted in the anime-only Pirate Dodgeball match, which comes with a massive rulebook with Obvious Rule Patches for seemingly every single thing that could happen during a match, no matter how farfetched or downright ridiculous (there's a rule for accidentally swallowing the dodgeball.)
A far less humorous example comes up far earlier on. In Nami's backstory, she makes a deal with Arlong; If she works as his cartographer, he'll sell her back her village and release her from service if she can raise 100 million beri. Years later, she's only 7 million away from that goal, but Arlong doesn't want to let her or her village go, so he has a corrupt Navy officer confiscate her stash. When Nami accuses Arlong of trying to break the deal, the bastard smugly retorts, "When did I break my word?"
CP9's treatment of the Straw Hats…or at least the way that their Jerkass leader does it. The deal they made with Robin was to let them go free from Water 7. When the Straw Hats come after them on another island, Spandam declares since they did leave Water 7 freely and choose to pursue them that attacking them then isn't breaking the deal. Also how they leave the Straw Hats is an example, though never pointed out. Sure they didn't kill the Straw Hats, but they did frame them for murder, throw two of them into the sea with a massive storm on the way, leave another buried under a pile of rubble in a burning building, and leave the last one with the people whose boss she had supposedly murdered.
In the third sound stage of Magical Girl Lyrical Nanoha, Fate is unable to contact Nanoha in real time because of restrictions placed on her for her trial, which is why they exchange video letters to see each other's faces and hear each other's voices. During a celebration of the anniversary of Fate making a contract with her familiar Arf, Lindy allows Fate to see a video link of Nanoha launching Starlight Breaker fireworks and Amy suggests that the video, lagging behind by 0.05 seconds, is not technically real time.
The TSAB limits how much concentrated power is allowed in a single unit to cut down on Interservice Rivalry. So how does Hayate get her family and best friends on the same squad when every one of them approaches or exceeds the power limit on their own? Have everyone wear Power Limiters of course.
Higa of Sekirei pulls this, bringing extra Sekirei to the third match, stating that it's okay as long as they don't fight.
In IGPX Immortal Grand Prix, on two separate occasions teams get away with doing things that logically should never be attempted, as there was no rule covering it. The first time, a team creates a tornado that was just as likely to get them killed as it was to win them the race. The second time, another team uses devices to take control of their opponents' vehicles. The rules are amended both times to stop them from repeated abuse, but they get away with their victories. The second team would also later reveal (in the final race) that their vehicles were actually Combining Mecha that could effectively block the entire track, but they lost that race (though the mecha blocked the track horizontally, Team Satomi made it past by using their own loophole: since there's no rule against jumping, they went vertical, leaping over the mech and dashing away, forcing it back into a straight race).
Yugi exploits an infinite loop combo that Marik/Strings used to fill his hand... to mill his own deck. (This stunt is not indicative of how Revival Jam and Brain Control actually work in the real card game, but this was often the case with the show's more memorable power plays.)
Similarly, the Legendary Dragons in the Doma Arc can do freaky infinite-loop tricks because they're already logistical nightmares when it comes to explaining how they work. Apparently, the way the Japanese "Dragon" Cards work is that any character who picks them up immediately gains a fusion deck about two feet deep to represent every possible fusion combo that the Dragon in question may form…or it could just be that the dragon's card morphs into whatever the Fusion monster is.
The rules of the Death Note say that you can't use it to cause murdersnote Specifically, you can't mention any other people in the Cause of Death or the victim will default to having a heart attack.; in the Live-Action Adaptation, Light gets around this by setting up two deaths so that the only possible way they could occur is if Person A commits suicide and accidentally kills Person B in the process. He wrote "Naomi Misora will take her gun and go to an abandoned part of the museum at 5:00 PM, then shoot herself six minutes later" and "Shiori will go to an abandoned part of the museum and get shot dead by a stray bullet at 5:05 PM". He also points out that Japanese gun control laws (that is, civilians are not allowed to own them) make it extremely unlikely that any other guns would be around to mess this up. Ryuk is so impressed, he tells Light that he'd make a better Shinigami than any of the ones he knows.
There's also a rule that says that after you write the name of the person you wish to kill you only have a certain amount of time to write the desired cause of their deaths (if you don't write a cause it defaults in a heart attack), there's nothing that says you can't write the cause first and leave a space for the name to fill in later. This is how Light tricks Raye Penber into killing all of the FBI agents investigating the Kira suspects simultaneously and thus averting suspicions being placed on himself by killing only the FBI agent he knows about aka the one placed to investigate him.
In No Game No Life, one of the Ten Pledges everyone in Disboard is bound to states, "Being caught cheating during a game is grounds for an instant loss". Sora quickly realizes this means that if the other person can't prove that you're cheating, you can cheat all you like and get away with it. Of course, he's not the only one who realizes this.
Both Leonard Apollo and Himura Youichi in Eyeshield 21 used loopholes to get out of their obligations and back into their respective countries. Apollo stated that if they failed to beat the Devilbats by at least 10 points, the NASA Aliens would never return to America. When they failed to meet that goal, Apollo revealed that he was just going to change the team's name to the NASA Shuttles; That way, it technically wouldn't be the NASA Aliens going back. Likewise Hiruma said that if the Devil Bats didn't beat the Aliens by at least 10 points the entire team would leave Japan. However he never said they would leave Japan forever...
Saitama Chainsaw Shoujo has protagonist Fumio observing that there's no rule against bringing a chainsaw to school, after the student council president tells her off for not switching to her school shoes. This being while Fumio is holding a bloody chainsaw.
Rocket Girls: Ain't no rule that says you can't send a high school girl into space!
In Urusei Yatsura, Mendō says that, although motorcycles aren't allowed at school, there's no rule against helicopters.
Invoked but defied in chapter 42 of the manga version of Shinryaku! Ika Musume. During a soccer match where Ika is performing poorly, her teammates realize the rules only say non-goalkeeper players can't use hands to move the ball. There's not a thing in the rules about tentacles (especially since Ika's tentacles are technically part of her head, which explicitly can be used to move the ball). But since Ika's trying really hard to play the normal way, they don't mention this loophole to avoid hurting her pride.
G Gundam has quite a few instances of nations finding loopholes in the rules. One of the most famous is the Nether Gundam; the rules say that any Gundam that goes through the Survival 11 without losing its head makes it to the finals. Neo Holland's Gundam turned into a windmill, spent the 11 months hiding without participating in a single fight, and qualified (didn't do very well afterwards though). Then there's Neo Nepal: the rules say that you can't kill your opponent during a Gundam Fight, but there Ain't No Rule that keeps you from going around assassinating your opponents right before your match...
In Beelzebub, when Oga is asked whether or not he's going to play volleyball with a baby clinging to his back, everybody agrees something to the effect that it's okay since the baby is a part of him. Later in the match, the Ishiyama team hilariously use Baby Beel to distract/trick the other team.
At least in Bokurano's anime, it's played straight twice in a row when Kokopelli asked Koyemshi to let him fight the last battle on their Earth and the demo battle on the next Earth so his also contracted daughter Yuu wouldn't have to fight. When Koyemshi stated it was impossible, Machi stated he could do just that because there is no rule that prevents you from becoming the pilot for more than one battle. Koyemshi retaliated: the rules are specific enough to state that you're still bound to the contract and keep being the pilot if you survive the battle without moving because the robot just sucks your life out to move. Kokopelli moved, so he technically doesn't have to be the pilot for the next battle. No points for guessing who was the one selected as the final pilot.
In Darker Than Black, only the Contractor's original body is obliged to fulfill their Renumeration which, obviously, doesn't have to be fulfilled if said body dies. Thus, there are two sorts of Contractors who can effectively obtain free use of their powers: Contractors with Grand Theft Me powers that lost their original bodies (such as Mao), or Contractors whose powers aren't actually their own (such as Hei).
The first example is (arguably) not Loophole Abuse. This is actually the rule; the final payment for a Possession contact is losing their own body, being forever trapped in the body which they transferred to when their body was destroyed. This is considered fully paying up their contract. Hei's Loophole Abuse, however, stands.
Heaven's Lost Property has Angeloids, All-Powerful Robot Angel Genies which programmed to do anything that their master tells them to, with the caveat that they cannot cancel an order once it has been given. That doesn't stop them from forgetting that their master gave a specific order, or making it so that whatever went wrong was All Just a Dream.
Done by virtually every team other than Team Urameshi during the Dark Tournament arc of YuYu Hakusho.
Digimon Xros Wars: The Young Hunters Leaping Through Time has an excellent example of this in one episode. A powerful Digimon is blocking all of our heroes' attacks with a seemingly impenetrable barrier. It's suggested that they bring out Shoutmon X4, the signature Digimon of the previous season, but the problem is that, in the current situation, the humans are forbidden from combining more than two Digimon at once, and cannot have more than one present and fighting at a given time. Since X4 is a combination of four Digimon, it would seem impossible to bring him out- but Taiki figures out how to circumvent this. He temporarily lends two of X4's components to teammate Yuu, then they perform the Double Xros maneuever, each using two Digimon, which enables them to successfully Xros Shoutmon X4.
Inverted, then played straight in Rinne no Lagrange: There's no rule saying that a student may pilot a Humongous Mecha... nor is there a rule saying they may not do so.
Ai no Kusabi has ruling Blondie Iason Mink making Badass Biker Riki his Pet. Something that just isn't done since Riki is a "mongrel", disenfranchised with no hope of climbing the social ladder and owning Pets indicate one's social status. Iason claims he doesn't have to follow the protocols for owning a Pet with Riki because he's a mongrel, has no official identity and thus the rules don't apply to him.
The latest arc of manga Katekyo Hitman Reborn! is a battle royal with one set of rule that really straightforward and simple it's a wonder why no team tried to exploit for seventeen chapters. Vindice exploit the loophole with attacking other teams outside of assigned time without wearing their representative watch and Varia also did the same though in their case Squalo just make a come back because they happened to have a spare watch and as lampshaded by himself, there is no rule about using the spare watch.
In Gintama, Gintoki once found himself poisoned at the very beginning of a glitchy virtual reality RPG, unable to walk far enough to get an antidote without dying, and not having anyone around to help him. His solution to the dilemma is to abuse the RPG rule that poison damage only takes place if you take a step by turning the coffin of the recently deceased Hijikata (Who suffered a Critical Existence Failure by stubbing his toe on a sign) into a makeshift dogsled using a length of rope in his inventory and a nearby NPC. He doesn't take into the account the random movement pattern of the NPC, however...
In GA Geijutsuka Art Design Class, Tomokane and Noda have prepared a game to play during lunch: Everyone blindly selects a slip of paper, and must spend lunch obeying the rule selected, e.g. "Exchange your favorite dish with someone else", or "Don't use sauce". Tomokane's was "Don't use chopsticks", so she decides to interpret hashi as "the ends" instead of "chopsticks", based on a different kanji.
Another time, the girls are playing "color tag" (the person who's it names a color, and if you're touching something that color, you're safe). Noda and Tomo get the brilliant idea to grab a color sample book, so the Professor (who's it) lists off the next color in CYMK format and tags them while they convert it in their heads.
When one of the Ooarai team's opponents tap into their radio communications during a match, Miho's team stops to check the rulebook and confirm that, indeed, there is no rule either way about doing so. They then go on to pull a similar exploit, communicating via mails sent between their cell phones and sending false messages over the radios.
Girls und Panzer der Film is full of this: Ooarai saves their tanks by officially claiming they "lost" them and having Saunders take possession in the meantime. When Ooarai is then matched against a pro team, outnumbered 30 to 8, all the other schools pull a "short-term transfer" into Ooarai and bring their tanks with them, evening the numbers. One of the match sponsors objects, but is told that since the schools' tanks are private property, there is no rule against bringing them along when transferring.
In Princess Tutu, the title princess is a duck who has been turned into a human girl so that she can restore a fairy tale prince's missing heart. However, if she ever confesses her feelings to the prince, she'll turn into a spark of light and disappear. At the end of season one, the Dark Magical Girl steals the prince's feeling of love with the intention of forcing a Heroic Sacrifice... but Tutu realizes that she can show how much she cares for him through dance rather than words.
In the first episode of Onee-chan ga Kita, Tomoya, annoyed at his sister Ichika constantly waltzing into his room, puts up a sign in front of his door stating that if she enters through here he won't talk to her anymore. So naturally she gets a ladder and tries to climb in through his window instead.
Puella Magi Madoka Magica ends with this: the wish that Madoka ultimately makes is to erase all witches from existence. Kyubey hopes this will result in the creation of a massive soul gem following Madoka's becoming a witch, but Madoka's power had become some immense that she was not only able to carry through on her wish, but also prevent herself from becoming a witch, becoming the concept of hope in process.
In Detective Conan, Vermouth promises Conan that she won't go after Ai and try to kill her. That doesn't mean that she won't let someone else do the job for her. The main reason why Ai is still is alive is because Vermouth hasn't gotten around her other, self-imposed promise; she isn't going to tell anyone that Ai is really a shrunken Shiho Miyano if it means that someone's going to logically figure out from there that Conan is really a shrunken Shinichi Kudo.
The girls from Nichijou should work on their rules stating when playing a game. Mai is prone to use loopholes.
One example is them playing red light green light. No one specified you have to turn again after red light.
Another is their stair climbing game. Yukko only bends the rules with a compound word, but no one specified you can't use absurdly long videogame passwords as the chosen word.
Though when the password is tried in the game later, it doesn't work...
Mobile Suit Gundam: The Antarctic Treaty that both sides of the war abide by is often summed up as "banning all WMD's". So some people get confused that the Solar System (essentially a giant space-borne version of Archimedes' Burning Mirror) or the Solar Ray (a giant particle cannon) are used without comment any comment of Treaty violations. Actually, what the Antarctic Treaty bans are "Nuclear, biological, and chemical weapons; and Colony Drops". Neither the Solar System nor the Solar Ray utilize nuclear power despite their destructive potential, thus they aren't banned by the Treaty.
Something of a meta-example concerning Gundam Build Fighters. The failure of Mobile Suit Gundam AGE caused one of the networks that had previously aired Gundam to cancel their contracts with Sunrise. As a result, the Build Fighters staff was legally barred from using any mecha from any shows that had aired on that network until after their contracts expired. They found little ways to get around it (such as having a Gundam Spiegel wield the AGE-1 Spallow's knife), but the big surprise game in the final episode, when Meijin Kawaguchi III shows up with a Gundam Exia R2 — which they were able to include because the network's contract on Mobile Suit Gundam 00 Season 2 expired two days before the episode aired. The staff tweeted that the brass were very upset over this.
An in-universe examples occurs during the third round of the World Tournament. During this round, each fighter is assigned a weapon completely at random, which range from extremely powerful to completely useless. During the battles in this round, only these weapons may be used. Mao, in particular, ends up with a rather useless paint gun, while his opponent gets a huge hammer. Mao simply blinds his opponent with the spray gun, then steals their hammer and knocks them out, cheerfully pointing out that the rules never said anything about stealing the opponent's weapon and using it against them, just that only those weapons were allowed to be used.
Gundam Build Fighters Try probably gives us the ultimate in Loophole Abuse: "Gunpla is freedom!" Strictly speaking, as long as its a Gundam or from the Gundam universe, your Gunpla can be anything. Except in the final episode, where Minato Sakai's entry into the Meijin Cup is disqualified because it's modeled after a real person, whose permission he didn't obtain (and who is clearly not happy).
In the second episode, Miyage attacks Fumina and Sekai with a Mobile Armor in their second match to decide the fate of the Gunpla Battle club. Fumina accuses him of cheating as you can't use a Mobile Armor with other Mobile Suits around in a tournament match. Miyage points out that this isn't a tournament match, so it's all fair game.
Daily Life with Monster Girl: The main rule of the Interspecies Protection Act is that humans are forbidden to harm monsters and vice versa... which enables such things as monster criminals freely doing as they please with the human police unable to intervene.
Although MON (a group of monster police) are under no such restrictions.
After Kirin goes back home in episode 3 of Koufuku Graffiti, she tells Shiina and Ryou not to eat any cakes without her. Shiina immediately suggests pancakes, stating that they're technically not cakes. However, Ryou then gets a text message from Kirin telling them not to eat any pancakes or crepes either, causing Shiina to let off an annoyed grunt.
Bobobo-bo Bo-bobo has an instance of this in the fight between J and Denbo. J Wouldn't Hit a Girl, but has no problem with fighting Denbo because "she" is a fusion between two male characters.
Kimihagu: Emily becomes the disciplinary committee president and forbids any romantic behavior on the school....however, there are no rules against A Date with Rosie Palms (and when Emily goes to the bathroom in Episode 2, well...let's just say that she gets a little carried away, all by herself).