Fantastic Fragility

Everything has a weakness, every lock has a key, every curse a loophole, and the flaw will come into play to bring down the device.

This applies to basically every "immovable object" and "unstoppable force" in fiction, be they objects, characters, science, or magic. The idea is twofold: inside the story, the creator of the device or technique may incorporate a Necessary Drawback that will make it stronger, while the author gives the protagonists (or antagonists) a way to defeat it and maintain suspense and narrative flow by limiting the Deus Ex Machina.

When Justified Trope in a setting, it's usually explained that Magic A Is Magic A is not free, and the only way to make an unstoppable whatever is to, ironically, build in a weakness that makes it stoppable. Clever creators may even keep the weakness a secret to use in case the machine (or person) should be turned against them. Others may try to make it an Impossible Task.

It can be a key to an Tailor-Made Prison, a physical weak point, not being able to harm virgins or the like, or a Curse Escape Clause. If the weakness is a Power Source, Soul Jar, or a form of remote control, destroying the "Keystone" destroys the device/weapon/person/army.

See Also Celestial Deadline, Necessary Drawback, and Power At a Price. Compare Achilles' Heel, which is this for creatures/characters.

Anime

 * The basis for much of the plot in Umineko no Naku Koro ni. Beatrice explains that, while she could very easily use her magic in ways that leave her utterly invulnerable, it is much more effective to leave the Ushiromiya family a chance (however slim) of successfully defeating her. To illustrate the reasons for this, a comparison is drawn between magic and gambling — the greater the "risk", the greater the "reward", so a sure chance of victory leaves nothing to gain. However, it's entirely possible that this isn't meant literally; Bernkastel claims that boredom is the only way to kill a witch, and it's very possible that the "no risk, no reward" paradigm is entirely psychological, as if they leave themselves no chance of losing a game, it is no challenge, and therefore "boring".
 * Nen abilities in Hunter X Hunter can be given a condition and a consequence to boost their power. The more restrictive the condition, and the worse the consequence if they break that condition, the more the technique's power is multiplied by. For instance, Kurapika's nigh-irresistable "Chain Jail" power has the condition "may only be used on members of the Phantom Troupe", and the consequence "automatic instant death".

Comic Books

 * The Green Lantern power rings have three. One, each ring must be recharged every 24 hours. The rings are weak against yellow. And finally, the ring requires thought and concentration to use. If you're tired or distracted, you're useless.
 * The weakness against yellow was originally meant to have been a built-in fail safe in case any one Green Lantern went rogue and tried to use the ring for their own gain.

Film

 * In Hellboy And The Golden Army, said army is completely indestructible and invincible. However, its greatest weakness comes from the Golden Crown with which it can be commanded. Disassemble or destroy it and the army becomes a bunch of clockwork paperweights.
 * The Fair Folk tried to counter this by stressing that the crown was completely indestructible. Effective in that nobody ever tried.
 * Curses and spells cast by the Big Bad in Disney works generally have an escape clause, although it's rarely stated why they're included.
 * Star Wars. "Just to make the fight fair, we put in a little hole that will blow up the whole moon-sized ship if it gets shot, K?"-Death Star engineers to Darth Vader
 * An unintentional oversight. The weakness the second time around was almost certainly intentional, after all, what better to bait a trap with than an easy victory.

Literature

 * Chronicles of Thomas Covenant.
 * The wild magic power of white gold can break the Arch of Time and release Lord Foul from The Land.
 * The Arch of Time itself, it prevents Loud Foul from escaping, and limits his power, but if the Creator tried to reach into The Land to stop Lord Foul directly, the Arch would be destroyed and Loud Foul would be released.
 * From The Other Wiki: During the Second Chronicles, the thaumaturge Kasreyn said that each of his theurgies contained a single deliberate flaw, because perfection could not endure within the imperfect world. He claimed that white gold would allow him to overcome this.
 * In The Hitchhiker's Guide to the Galaxy book Life The Universe And Everything, the population of the planet of Krikkit is in a Slo-Time envelope. However, a separate key (the Wikkit Gate) was first created and then dismantled so that the envelope can be undone after everything else in the universe is dead (so that the Krikkiters can have their wish to be the only things in the universe). Wasn't a problem until they sent out the robots.
 * The Lord of the Rings. The One Ring is completely indestructible except for either being thrown into the fires of Mount Doom from whence it came, or dismantled by a smith more skilled than Sauron.
 * Even then, it'd take several centuries, and the matter's a bit more pressing.
 * The Hobbit. Smaug the dragon is viewed to be nigh-invulnerable by everyone who knows of his existence, including the dragon himself. His scales form a powerful armor in and of themselves, and what would normally be a soft belly is covered by innumerable hard gems embedded in his flesh from years of sleeping atop Dwarven treasure. There's just this one little exposed patch, but you'd need sharp Hobbit eyes to make it out in a dark cavern. And come on, what are the odds of a Hobbit being anywhere near a dragon cave?
 * In Terry Pratchett's Sourcery, a wizard's attempt to cheat Death must, by the rules, include a condition under which the contest will end. The wizard in question at first suggests "when Hell freezes over," but is told that this would break a separate rule that forbids living people to be told what the afterlife is like.
 * In Stationery Voyagers, Whixtitians who use Mikloche powers can do a ton of really cool things. Anything from flight and teleportation to becoming a one man army with microwave beams coming out of every orifice of your body. The catch? Three catches, really. 1) The power is unlocked by "security shells" which are shed almost like snakes shedding their skin. Promotion is literally agony. 2) The more security shells you break through, the more at risk you are of suffering brain damage or Superpower Meltdown. Killing you quite violently. 3) Mikloche flight doesn't protect your head from air friction. So without a helmet, you'll fry your brain if you try to keep up with a commercial jet.
 * Angels are allowed to get away with a lot in this universe. But at the expense that they don't have as much free will as they used to. Volition Dilemma is appeased for them and can no longer endanger them; but they are now enslaved to their Character Alignment.
 * Astrabolo can wield a lot of influence over his cult members, but cannot negate Volition Dilemma completely since they'd have to die on him in order to appease it. To do otherwise would require defeating the Definition Essentials that even dictate the nature of God. (A source of constant frustration to the villains, and a recurring plot point.) But doing that could cause the universe to implode.
 * Due to artificial reincarnation, Mechanical Pencils can survive for centuries, and get new bodies if they wear out or trash their current ones. However, they're useless without a nuclear fuel source. Depleted Mechies find their spirits trapped in a sort of Limbo until they're inserted into a new body that has lead balzhite fuel.
 * The power of any wizard is contingent on how much his power source (the Dark Wanderer) can one-up Levio the Nullifier any given minute; making them about as reliable as a freeloaded Wi-Fi connection from long distance at best. Unless they combine their powers with technology.
 * Mosquatlons, like other types of vampires, have to avoid the sun. But, they get a lifespan eight times that of humans and age accordingly. You can look like you're smokin' hot and in your early 30's when you're really 300.
 * The Demon Prison Zzyzx in the Fablehaven series was constructed according to this principle. If the magicians had attempted to make it utterly inescapable, the magic would have weakened and inevitable failed. By leaving the loophole of the keys, the magic may be kept strong, although the keys must then be defended. Destroying them would result in them reappearing randomly sometime later. To their credit, the architects really did just about everything to make it as hard as possible to successfully unlock the prison as possible.
 * In Esther Friesner's Elf Defense, an elf explains that "only the Infinite is infinite" -- which means anything not the Infinite has to have a weakness. (Specifically in this case, an elven vulnerability to Latin.)
 * In The Book of Swords, Shieldbreaker is the greatest of all the Swords. It can destroy any weapon brought against its wielder. However, it is completely useless against an unarmed opponent, as such an opponent has no weapon to destroy..
 * In the Tales of the Five Hundred Kingdoms series, all curses and such have this. If they don't, the Tradition will put one in.

Live Action TV

 * In Doctor Who, the Sonic Screwdriver can do anything with any device. Unless it's wood or "deadlock sealed". It also doesn't seem to work too well on stone.

Tabletop RPG

 * Dungeons and Dragons. In earlier editions, artifacts and relics could only be destroyed by a single specific means.
 * And in Ravenloft, a curse with an escape clause is much more likely to take.
 * The New World of Darkness likes this one.
 * Werewolves can make traps etc. for spirits but they always have to include a way out.
 * Changeling: The Lost's are taught that everything has a price but nothing is truly forbidden; this comes to the forefront with fetches, some of which have a catch called Fragile Creation in which one specific thing will destroy the fetch utterly - but nothing else will.
 * Vampires gain new powers only if they gain new weaknesses, probably how the whole immortality vrs sun thing happened.
 * Prometheans can do all kinds of nifty stuff with their bodies, except the power that animates them is utterly unnatural and the universe acts accordingly.
 * Mages have phenomenal cosmic power... but are still squishy wizards. They are also loathed by the universe which allows paradox to do nasty things to them.
 * Sin-Eaters have it relatively easy though; they get brought back from the dead for the low-low price of a kinda-sorta Enemy Within urging them to indulge in The Dark Side by fulfilling all its old vices, which is often told to shut-it.
 * Spirits can gain new powers by taking on a Ban, which can lead to a being that can eat souls but is flummoxed by salt.
 * In Exalted, the can holding the Cosmic Horror Yozis prisoner is the body of their own king, Malfeas. Their imprisonment was deliberately made imperfect (allowing the lesser Legions of Hell to escape sometimes) for fear of what the Yozis would have done to resist being imprisoned if it had been perfect. (One Yozi's petty and petulant response to imprisonment destroyed roughly 2/3rds to 90% of all reality as it is!)
 * The Exalted themselves have perfect defensive charms that can make them invulnerable, but which always come with some sort of situational or tactical flaw. For example, a Solar invulnerability might only work in the presence of someone they care about, or might force them to advance on their most powerful opponent. For Abyssals, their invulnerability might fail in the presence of someone they care about, or force them to flee their strongest foe. For the Infernals, their perfect defenses possess a flaw based on the Yozi patron that grants it. Thus, invulnerability charms granted by Malfeas the Demon City only function in a developed area, while charms from the Ebon Dragon, made from the shadows of everything in existence, cannot defend against holy attacks.
 * Though it's touched on in the novels of The Dresden Files, the RPG makes it explicit: If you want to be Made of Iron, have a Healing Factor or just be Nigh Invulnerable, you have to have a Kryptonite Factor called "The Catch," which, depending on the availability, gives a discount to powers from those power sets. Having one that's well known and abundant, such as iron to The Fair Folk, gives a bigger discount than one that's obscure or hard to get a hold of.

Video Games

 * Alexander comments on this trope in King's Quest VI during the Beast scenario.

Western Animation

 * According to Word of God, magic in the Gargoyles universe is more powerful if you give it some sort of get out clause (even an absurd one). Hence the series' premise that the petrification spell on the Gargoyles was undone by the criteria "until the castle rises above the clouds."
 * A regular gimmick on the show. The aforementioned spell is broken in modern times when the castle is moved stone by stone to the top of a skyscraper. Another spell is set to end "when the sky burns" - this condition is met by having robots spread a layer of combustible gas in the atmosphere over the city, then igniting it. Demona and Macbeth have a spell of immortality on them - they will live forever, unless one of them kills the other.
 * In Justice League Unlimited, Hephaestus creates an invincible, sentient suit of armor for Ares, but like all his creations, he builds it with one fatal flaw that he keeps secret if it were to be used against him (or possibly because Zeus considers it blasphemous for perfection to exist in anything except gods): the suit is powered by conflict, so if nobody is fighting around it, it shuts down, which may not sound like much of a weakness until, say, someone throws rocks at you from 10 miles distant, too far away to activate the amour.
 * This weakness is removed by Felix Faust when he uses it as his new body. Fortunately Hawkgirl's weapon is the Kryptonite Factor for the suit.
 * Wonder Woman's armor apparently has such a weakness too. When she goes to ask Hephaestus directly how to stop the Annihilator, he refuses to tell her, asking if she would want him to tell people about the weakness he built into her own gear.