In Last Second SaviorHarry and Aayla Secura have a Time Turner with just enough sand to send both of them back nine hours once. Harry warns Aayla that they absolutely can't change anything they know for a fact happened (like saving Barriss Offee). However since they don't know for a fact who else died, they can send a warning to the Jedi Temple and have it forwarded to every Jedi except Aayla and Barriss.
In the Jackie Chan Adventures and Teen Titans crossover fanfiction A Shadow Of The Titans, Jade does this in a Cutting the Knot type situation. Basically the Drill Sergeant Nasty gym teacher for the HIVE (an overweight and balding villain with No Indoor Voice called Captain Hollywood) wanted her to run a dangerous obstacle course and ring a bell at the end. Jade asks again to make sure that the goal is to ring the bell, and then the timer starts, prompting Jade to turn around, since the course was circular. The school considered it valid, but Hollywood still sent her to detention, since the HIVE is a villain school and you don't need to break the rules to get punished.
Karril: Hook, line, and sinker. Rassilos said - and I quote - you think we'll believe that Gerald Tarrant, the Darkest Prince of Hell, cares what we get up to?'. He thinks I was only fear-mongering. That covers the requirements of forewarning, though, so you're free to slaughter them at will.
In Uzumaki Harry the Four Tri-Wizard Champions point out no one said they had to compete against each other, the official rules say only to participate, as well as the fact that the tournament was originally formed to forge bonds between schools—so they are totally justified in working together.
Many Death Note fics will circumvent the first rule of the Death Note "the human whose name is written in this notebook shall die" by revealing that a character isn't actually human but a Shinigami or a vampire or a werewolf or an alien or whatever.
In Desideratum L is immune to the Death Note because he doesn't have a real name.
In Haunted Light figures out that Rem can't carry out her threat to kill him if he kills Misa because Ryuk has to be the one to write his name when he dies; so Light kills Misa anyway and there's nothing Rem can do about it.This loophole is actually canon-it's Rule 34note No, not THAT one of the Death Note: "1. The owner of the Death Note cannot be killed by a god of death who is living in the world of the gods of death. 2. Also, a god of death who comes to the human world, in the objective to kill the owner of the Death Note, will not be able to do so. 3. Only a god of death that has passed on their Death Note to a human is able to kill the owner of the Death Note."
In The Killer In You Rem is able to get around the restriction that Shinigami aren't supposed to help humans by giving them the names of people they want to kill by giving Light the names in 1337 lingo that she apparently learned from watching Misa chat. Because she writes it as "'Ryuzaki: L L a 2u l i e t, 2uatari: Q u i l l s h 2u a m m y" she didn't technically give the name.
From the AU of Ragnarok after learning that Light is secretly the legendary hacker known as "Kira" (that the NPA pays a lucrative bonus for his consultation) L complains when Light again hires "Kira" to aid in his investigation. Light has this to say for himself:
Light: Technically, Kira is Matsuda's source, not mine. I have never stated that I was not, in fact, Kira. Technicality is the soul of the law.
In the Discworld Tarot chapter The Ace of Swords, Miss Alice Band is able to tame a rogue unicorn where others have failed, despite being sexually experienced. The reason Alice gets away with it is because her sexual experience is only with other women. She still technically qualifies as a maiden, unsullied by contact with men, and therefore meeting the specification for unicorn-wrangling - because she has never had contact with men. It is possible that while the unicorn was trying to make its mind up as to whether she qualified, she got a silver-ornamented bridle over its head and settled the question definitively.
In 3 Slytherin Marauders the Goblins inform Tom Riddle that he can't completely empty out Voldemort's Gringott's vaults... so he leaves him a single Knut. Also fugitives can seek asylum at Gringott's because "Gringott's customers are allowed to stay and count their gold for as long as they like."
Demons tried to kill their summoners because they despised humans generally but even more the ones, who dared to force them under their will. Which demon wanted to serve a mere human? So they tried to interpret the commands of their master in such a way, that allowed them to hurt or even kill their caster. If the summoner wasn't careful enough, that would happen very quickly. The demons had to do whatever the summoner demanded, but if he didn't word his demand carefully, the demon would take revenge later. For instance, one of the inexperienced Necromancers in the past had summoned a Demon. He had worded his demand carefully but had forgotten to forbid the demon to bring anything with him. So he brought two things, a burning torch and a barrel full of black powder. He didn't hurt or kill his summoner directly, but as he swept down back to hell, he let both fall down to the ground and the Necromancer was... well... history.
In the first match with an explicit rule, namely you're what ever faction you state you are, which means points are taken away from that stated faction if you are knocked (which allowed for spies and the massive Gambit Pileup that happened in that match), Harry joined forces with Draco's army, with both of them declaring themselves as part of Hermione's army, meaning she lost the massive lead in points.
Given that the main character of Harry Potter And The Natural 20 is a huge Munchkin from an RPG Mechanics Verse, this is to be expected. One particular example is how he handled the problem of obtaining resources to make magic items (which require a sacrifice of expensive material): buying up massive quantities of table salt. A pound of salt is worth only a few cents in the modern world, but it's worth five gold in the world of Medieval Stasis.
In Harry Potter and the Parseltongue Legend the Hogwarts rules state that students are only allowed to bring an owl, cat, or toad, or a bonded familiar. Since Harry already underwent the familiar bond ritual with his magical cobra he's allowed to keep his snake.
The premise of the one-shot Slash FicI bid him! Harry makes use of the old medieval law of "Verbidden" (in which a criminal facing the gallows is released in exchange for marriage) to save good!Voldemort from execution and since the law was drafted in the middle ages and homosexuality was considered abnormal it was just assumed that it would always be a woman so no one specifically wrote the law so that it had to be a woman who bidded someone.
In Impossible Harry utilizes this after being forced to compete in the Tri-Wizard Tournament—he enters the arena with the dragon and instead of bothering the dragon or trying to get the golden egg he summons Quidditch Through The Ages and conjures himself a comfy chair and just sits there reading for twenty minutes:
Dumbledore: Mr. Potter. What, may I ask, are you doing?
Harry: I'm doing the task.
Dumbledore: But the task is to collect the egg from dragon.
Harry: No. To complete the task I would have to get the egg from dragon however just entering the arena constitutes competing which is enough to satisfy the contract.
In Liquida Tenebris Harry gets Voldemort to vow on his life and magic to stop touching him. Voldie swears... and then immediately starts poking him with his wand.
In Lord of Caer Azkaban Harry promises to remain at Hogwarts until he's completed his NEWTS... and then he hoodwinks Dumbledore into letting him take his NEWTS early.
In The Mind's Guardian, an AU in which Harry has allied himself with Fudge against Dumbledore, as much as Fudge doesn't like Dumbledore he doesn't see any way to legally be rid of him but then Harry finds a loophole:
Harry: Isn't perjury grounds for impeachment?
Norton: You're referring to him claiming Voldemort's returned? Unfortunately, speeches before the Wizengamot and ICW are not sworn statements during a judicial proceeding. If he were anyone else, the Wizengamot might hold him in contempt, but that's all anyone can do.
Harry: I wasn't referring to his 1981 speech. I meant his testimony in the January 1982 trial of Severus Snape. Hogwarts doesn't have full transcripts, but The Daily Prophet printed an excerpt. I may be remembering this incorrectly, but I believe Dumbledore used the phrase 'Voldemort's death' during his testimony. If he's running around now saying he always knew Voldemort survived then he perjured himself during Snape's trial.
Dobby: Dobby wasn't trying to save the Great Harry Potter's life nor was he trying to send him away from Hogwarts. Dobby was trying to break Harry Potter's arm.
In Past's Player Harry promises to stop Obfuscating Stupidity if Tom Riddle will just leave him alone. Tom agrees because Harry didn't specify how long he had to "leave him alone." Feeling generous he gives him a week...
In Pavement Diaries Bellatrix manages to steal the prophecy orb that only Harry Potter and the Dark Lord can touch:
As for how she left with the orb, It was such a trivial loophole. So trivial, yet so brilliant. Bellatrix couldn't pick up the orb, but she could move the shelf it was on. So she did, and levitated it in a bag complete with featherlight and invisible extension charms, and walked right out the front door with it.
In Saving Her, Saving Us All, Sirius is restored to life because after being sent through the Veil of Death, it turns out to be a Veil of Judgment instead, and it judges Sirius' soul to be "pure," and thus refuses to condemn him. Harry is of course tremendously relieved, but then Harry remembers how Sirius almost got Snape killed with a schoolboy prank, so in chapter 3, Harry reluctantly asks Sirius why the Veil judged him to be pure in spite of that. Sirius answers that it was because he was so dumb as a schoolboy, he genuinely-within-his-heart didn't know that his prank could have killed Snape, so the prank didn't dilute his "purity" to the Veil of Judgment.
Averted in Visitor when Voldemort draws up an iron-clad secrecy contract to prevent Poppy from mentioning her treating Harry Potter or anything about their meeting to Dumbledore.
"It really was as loophole-free as he'd said it would be. It really was a perfect secrecy contract. If she had actually been planning to tell Dumbledore or anyone else anything, this would have made it impossible. Even the method of 'I'm lying to you but making it so obvious that you know I am lying' had been included in it. When she was finally through it all, twenty minutes had passed. Taking the quill she was being offered, she signed her name, and the contract shone bright silver before disappearing."
Hivefled: The Condesce was permitted to choose her weapons for her battle with her ancestor. Technically, a lowblood follower counts as a weapon; she chose the Handmaid, the most powerful psychic ever known.
In And The Meek Shall Inherit The Galaxy, the Exact Words method of loophole abuse gets lampshaded and subverted when the newly formed Independant League is held to the galaxy-wide Dreadnaught limit. Some turians are furious about this due to the fact that the League willingly accepts the limitation... because the combat doctrine the humans introduced has all but made dreadnaughts obsolete in favor of carriers. The Citadel Council refuses to place a similar limit on carriers to stay on the good side of the League.
Hyouta Hyuga in A Growing Affection asked Naruto to agree to set aside his weapons so they could fight on even ground. When Hyouta breaks out his holdout kunai, everyone gets mad at him for breaking the deal, and Hyouta points out that he never agreed to fight without weapons, he just asked Naruto to do so.
Tsunade promised not to send any Leaf Shinobi to rescue Princess Nyoko. Which does not cover hiring the Sand Ninja to find the princess, and then telling a group of ninjas, who happen to be Nyoko's friends and have a history of going AWOL, exactly where she is being held.
New Chance: During the Chunin Exams, Naruto simple solution to the first test is to summon clones on the outside of the classroom (he's by the window) in full view of all the proctors. The clones proceed to work out the problems in the test by literally acting out the scenarios from the test.
Proctor: "Is he allowed to do that!? Does that count as cheating?"
Ibiki: (amused) "No. The clones still count as Naruto and since they're merely working out the questions outside of this classroom without disturbing anyone in here it doesn't count as cheating."
Real Skill has Kakashi sent back to the Academy after Naruto takes both bells during the bell test. As Naruto pointed out (and the Hokage agreed), the rules stated that anyone without a bell at noon would be sent back to the academy.