Kaolinite was instantly killed by Mistress 9 in Sailor Moon upon her revival. Likewise Metaria possessed Queen Beryl in the end of the first season.
Emperor Pilaf, the comic relief villain from early Dragon Ball, ends up releasing evil incarnate Piccolo Daimaou to get revenge on Son Goku. Piccolo proves infinitely more competent than Pilaf ever was: not only does he kill several of Goku's friends, but he kills Shenlong, the show's resident Get Out of Death Free Card. Considering there's 300+ episodes afterward, though, rest assured: he gets better. Pilaf and his minions end up punted out of a moving airplane. Initially, it seemed as though they had learned from their mistakes; something like 300 episodes (approx. 40+ years in show) pass before they reappear to steal the Black Star Dragonballs... which ends in the entire planet blowing up. (Again.)
In Dragon Ball Z, Dr. Gero releases Androids 17 and 18, hoping to be able to control them with a remote control which 17 promptly takes away from him and breaks. A somewhat unusual case in that Gero was the one who created the Androids in the first place, and knew that he was taking a big gamble when he activated them. Although in that case would "Anti Villains are also not a toy."
Babidi revives Majin Buu and plans to use him to take over the world. Things don't go as planned.
Initially, Buu does obey Babidi and shows no interest in betraying him, probably because he's too stupid to think of such things. Even when Babidi insults Buu and threatens to re-seal him, the clearly enraged Buu backs down. Until Goku points out that Buu is far stronger (and faster) than Babidi and shouldn't take such abuse from him. Then Buu kills Babidi with a single punch to the face (which obliterates his head). Whether it was a good idea on Goku's part is rather questionable given Buu's later rampages, but Goku isdeeply stupid anyway.
Also, the immediate consequences of Buu betraying Babidi are largely positive, since his capricious frolicking leads him to accidentally discover that Good Feels Good. Until a vigilante tries to kill him and nearly offs his new best friend in the process. Cue Enemy Without.
In Slayers, the original Rezo found out that he would have to release Shabranigdo in order for Rezo to be able to open his own eyes; he believes he can adequately contain the lord of all demons thereafter, seal him back up, no harm done, right? It doesn't work out that way, mainly because Shabranigdo was actually sealed inside of Rezo; the very act of opening his eyes breaks the seal and allows Shabranigdo to take over his body.
Then there is Phibrizzo/Fibrizo, who arm-wrings Lina into casting the Giga Slave, in hope that the released power of the Lord of Nightmares will consume the world. He does partly succeed... the Lord of Nightmares itself is released inside Lina's body. It then promptly obliterates Phibrizzo.
The manhwaPriest uses this in the climax of its several volume long flashback; a corrupt order of priests have succeeded in opening the Domas Porada, the "can" (and it does rather resemble one) containing the fallen angel Temozerela, believing him to be their savior. Unfortunately, Temozerela isn't too fond of humans...he kills almost the order with a single breath (he breathes at them and their heads explode), mocks the leader a bit, and them kills him by making demon faces sprout out all over the leader's body and bite him to death.
Hell Girl both subverts it and plays it straight. Ai can be summoned by humans through her website in a fairly simple manner and will kill the person she's ordered to as promised. However, as she warns you, the cost of having her do this is that you get dragged down to Hell upon your own death too. Also, over the course of the series, certain people make attempts to capture and/or control Ai for her abilities or to avoid the cost of her services, but that never works.
In Yu-Gi-Oh!, Pegasus takes ancient magic duels that nearly destroyed the world and resurrects them as a (supposedly non-magical) card game, thereby threatening to incur the wrath of the Egyptian God Monsters. No wonder the game soon became Serious Business. (After said gods were placated, the game continued to be a Weirdness Magnet for all sorts of mystical forces.)
The Seal of Orichalcos is another example. As many villains of the DOMA Arc discovered, this cursed card does not play favorites.
In Yu-Gi-Oh! GX, both Fubuki Tenjoin (Atticus Rhodes in the English dub) and Yusuke Fujiwara seem to have trouble grasping this concept and end up using a Mask of Power and developing (the same) Super-Powered Evil Side. Then there's Brron, Mad King of Dark World, who tries to defeat Juudai by forcing him into despair by killing his friends. This activates Juudai's Super-Powered Evil Side, and does not end well for Brron. Ryo Marufuji/Zane Truesdale developed heart problems after forcing his opponents into duels in which both players wear collars that release electricity whenever the player takes damage.
Umineko No Naku Koro Ni: If setting up a family meeting just so you can sacrifice the whole lot of them to a witch to bring her back to life seems like a good idea, you probably need your head examined. Appropriately enough, Kinzo tends to be found dead on a regular basis. However, it turns out to be a subversion; he's been dead for more than a year.
This trope is a common theme in Chrono Crusade, usually having to do with the demons. The most obvious examples are the New York mobsters that think they can use demons as hit men, and Rosette's brother Joshua, whom accepted a Deal with the Devil to have a pair of demon horns on his head. Subverted with Rosette and Chrono—Sister Kate (and others) believe that Chrono will turn on Rosette, but it turns out he's not evil, but loyal and protective of her.
In Monster, various shady forces- a conspiracy or two consisting of Neo-Nazis, mad scientists, and ex-Secret Police amongst others- all want to use Johan to bring about their One World Order. Johan is also implied to be The Antichrist. For real. As Tenma points out, he is not interested in ideas of racial or ethnic superiority, for he "laughs at all of humanity". Ultimately he plays along with their plans for his own ends- namely, The End of the World as We Know It-, only to suffer an existential crisis and murder them all as a kind of metaphorical "suicide", vis a vis erasing everyone and everything who has any significant connection to him, while planning to be shot by Tenma. The last part fails, but the conspirators' plan never had a hope.
In Bleach, relying on the Hogyouku for power proves to be Aizen's undoing. Even though he survives Ichigo's Mugetsu and is on the verge of attaining even greater power, the Hogyouku is so unimpressed with his performance in the past few chapters that it no longer considers him a worthy master and depowers him. This activates a kidou Urahara hid inside Aizen which turns him into a Sealed Evil in a Can.
In Naruto the First Hokage used his Wood Release techniques to capture several of the tailed beasts and divided them among the other shinobi villages both as peace treaties and to stabilize the balance of power. However, being monstrous embodiments of chakra energy, the villages had trouble just containing them and attempts to utilize their power often met with mixed result (like Gaara and the Shukaku).
But completely averted with Madara, who was able to use his sharingan alone to bind the Kyuubi (the most powerful of the tailed beasts) to his will and attack both the Hokage and the Hidden Leaf Village separately (along with controlling the Sanbi and its human host in a flashback). His Evil Plan involves using the seven beasts he already captured to make 100,000 Zetsu from their chakra, with the overall goal of combining all the captured tailed beasts into one colossal ten-tailed beast, become its host, and use its power to place the entire world under his control with his Tsukuyomi technique.
Also averted by Killerbee, for whom the more appropriate trope would be Evil Is A Toy And Also Your Best Friend. He managed to become allies with the Hachibi, while Yugito appears to have had a decent relationship with the Nibi, which means that it makes a bit more sense for others to try it, too.
To his credit, post-time-skip Gaara was shown to be more or less in complete control of Shukaku by the time he became the Kazekage. Whether he had made peace with it (like Killer B) or had simply worked up the willpower to completely suppress its murderous tendencies wasn't revealed before the Akatsuki ripped it out of him.
During the Fourth Shinobi World War, Naruto actually has a talk with eight of the nine beasts and discovers that not only are they all individual sentient beings with varying personalities, they were once all pretty nice. Once the Sage of the Six Paths died, however, people began treating them like monsters and they became monsters in retaliation. With Naruto treating them like people, they all offer him their support and he's able to break Kurama/the Kyuubi's seal and let him share his body as an ally. So it turns out to be Evil Responds Well To Being Treated Like A Person.
Even Uchiha Madara admits that the Ten-tails might be difficult to control as it mutates.
In Star Driver, Glittering Crux gets a taste of this in episode 14 when their Maiden seeking Cybody Ayingott turns out to be an evil robotic horror that overpowers its own Driver and goes berserk. It immediately attacks the Crux leaders. Only Takuto's and Sugata's intervention prevents things from getting worse.
In chapter 242 of Fairy Tail, Zancrow gloats about retrieving Zeref. Seconds later, he is slain by a burst of Zeref's uncontrollable "Death Magic".
Hades wastes no time following his example. While trying to get the proper tools to undo the seal, Zeref informs him that he's wasting his time, that he has no interest in helping create Hades' magic horror world, and also that Hades trying to drag him into the whole thing has angered him enough to intentionally kill him.
Alan Gabriel in The Big O learns the hard way what happens when somebody thinks the Megadeuces are just a type of Humongous Mecha. Ye Guilty.
In episode 3 of Soul Eater, a witch revives an ancient pharaoh and says he is now her servant. The pharaoh angrily eats her alive and goes back to sleep.
A lot of Hellboy's enemies seem to think that they can use the Right Hand of Doom and Ogdru Jahad for their own ends (even Hell), while it is implied that they would merely destroy all of reality.
He actually created Dark Phoenix. Jean was a bit Anti-Hero-ish as Phoenix and worried about Jumping Off the Slippery Slopesomeday, but we don't get to wiping out solar systems until Jason Wyngarde's tampering with her made her dark side fully overcome her control, and suddenly Evil Feels Good reigns. It's all his fault.
In one Witchblade/Tomb Raider crossover comic, a haughty socialite goes to great lengths to summon the goddess Bastet, without ever considering whether she could actually control a divine being. When she wishes to be kept out of prison, the goddess remarks that this is easily arranged, and sics a pet lion on her.
Doctor Strange used the power of the demonic entity Zom to bolster his magics on at least two occasions. Both times the power used him far more than he used it, and after the second time he renounced both it and the mantle of "Sorcerer Supreme." He didn't get his title back until years later, once he'd demonstrated that he could use dark magic without letting it get out of his control and without hurting anyone else.
In Batman RIP, when the Black Glove attempts to use the Joker to finish off Batman. Joker himself tells them that they haven't come nearly as close to beating Batman as they think, and that they can't control the Joker himself. Later, after the dust has settled, Batman's won, and the Black Glove members have gone home to hide behind their money, Joker murders them all.
The My Little Pony: Friendship Is Magic fanfic Whispers implies that Princess Luna transformed into Nightmare Moon by intentionally tapping into evil magic out of bitter jealousy, apparently overestimating her ability to handle it.
Chaos: How is it that I'm supposed to do your schoolwork? I'm CHAOS!! Not some sort of encyclopeadia!
Films — Animated
In Disney's The Return of Jafar, Abis Mal gets pressed into servitude after accidentally summoning the genie Jafar. Jafar accomplishes this by constantly threatening violence against Abis. (The hapless thief is aware that in his world, genies cannot kill, but Jafar keeps repeating ominously, "You'd be surprised what you can live through.")
Also, Jafar is the epitome of the Jackass Genie, so it's not like he can make his wishes and be on his way.
In the 2009 Astro Boy movie, Stone is very enamored of the explicitly evil Red Core. Experience fails to teach him that you must not play with it.
In Marvel's Hulk VS animated movie, Loki in order to defeat Thor removes Bruce Banner from the Hulk and then uses magic to try and control the Hulk. This doesn't last long as the Hulk's rage soon overcomes Loki's control and the Hulk runs around destroying all of Asgard.
Black Mask: I'm being forced into negotiating with a psychotic.
Ms. Li: That doesn't sound good.
Black Mask: No. It's going to be a nightmare.
In Batman: Mask of the Phantasm, Sal Valestra enlists the Joker's help as a hitman against Batman, whom he thinks his killing off mob bosses. The Joker promptly murders Valestra to use as bait for the hit he himself ordered.
Films — Live-Action
In Hellraiser, Frank Cotton releases four demons with the magical cube, which looks quite like some toy. They don't exactly reward him.
It's neither a reward nor a punishment. The Cenobites' job is to bring the summoner to the heights of "pain and pleasure", at least in the first film. The summoners never really understand what they're getting into.
Probably even more true for Hellbound: Hellraiser II where Dr. Channard's obsession with the Lament Configuration ultimately leads to him becoming horrifically tortured and turned into a Cenobite himself. He's horrified at first, but in the end...And to think, I hesitated.
In Turbo: A Power Rangers Movie, the villainess Divatox wants to resurrect the demon Maligore and marry him to gain untold power. When she does, he starts attacking everyone, and is ultimately destroyed by the Rangers.
In The Mummy Returns, a museum curator, Hafez, deliberately digs up and raises Imhotep, thinking that Imhotep would defeat the Scorpion King. The fact that this would lead to an evil undead ruling the whole world apparently didn't cross the curator's mind. That or it did and Hafez figured Imhotep would let him help rule the world.
Actually, it might have something to do with the fact that the Scorpion King was going to bring about The End of the World as We Know It - Hafez probably thought Imhotep ruling the world would be better than it being destroyed.
In Fido, the kid takes his pet zombie to the park to play catch. When the collar controlling its zombie nature malfunctions, it eats an old lady and nearly starts a new Zombie Apocalypse. The adults of the movie barely punish him for this.
At least partially justified in that it's less the adults letting him off with a slap on the wrist, and more them acknowledging that there really wasn't any way Timmy could have predicted that Fido's collar would malfunction, nor could Timmy have controlled Fido, even if he had been able to predict the malfunction. And also, they were acknowledging that while Timmy had screwed up, he'd at least tried to fix his mistakes. Notably, by going out and killing the zombified old lady (by decapitating her with a shovel!), and then burying her. Granted, she'd already killed someone by then - but he couldn't see the body from where he found her, and it's implied that his knowledge of how long it takes someone to go from 'dead corpse' to 'zombie' was a bit fuzzy. Plus, the truth behind how someone died without Zombcon finding out shortly after was covered up; any open punishment they could have given Timmy would have brought the cover story into question.
The collar also wouldn't have malfunctioned if Mrs. Henderson hadn't been repeatedly hitting Fido with her walker. Really, she brought it on herself.
In Pirates of the Caribbean: At World's End, both Barbossa and Beckett are convinced they have what it takes to keep evil on a leash and working for them. First, there's Calypso, a vicious pagan goddess bound in human form by pirate lords, who Barbossa — a pirate lord himself — disrespectfully throws in the brig, with the intention of unleashing her on Beckett. Then, there's Davy Jones, Calypso's ex-lover turned heartless Reaper, who Beckett has enslaved and intends to have take on the entire Brethren Court for him. This, inevitably, works out for neither of them.
The Librarian: Curse of the Judas Chalice - A group of former KGB agents, wishing to restore the Soviet Union, try to resurrect Vlad Dracula in order to create an army of invincible undead soldiers. They hope to control the infamous vampire by using the Judas Chalice, but it's revealed that the handicapped vampire-obsessed professor they brought with them was in fact a disguised Dracula, who proceeds to feast on the men and turn them into his vampiric minions.
In Freddy vs. Jason, Freddy provokes Jason into rising up and attacking the teens of Springwood, in order to stir up a fresh generation's fear and give himself the power that'll let Krueger resume killing also. It works, but the two killers wind up sparring over a dwindling supply of victims rather than rampaging freely.
Dr. Cocteau in Demolition Man brings convicted criminal Simon Phoenix out of cryogenic imprisonment and gives him all sorts of Crazy Awesome weapons and combat training so he will kill rebel leader Edgar Friendly. Since Phoenix is already an Ax Crazy terrorist and mass murderer, this ends badly for Dr. Cocteau.
And in Batman Begins, Falcone works with Crane and Ra's al Ghul for the drugs they smuggle, unbeknownst that the latter two are actually aiming toward higher, more destructive goals. In addition, Crane thinks Ra's is holding the city ransom, not planning to destroy it without prompting as the case really is.
In Dracula II: Ascension, a group of idiots (to call them scientists would be charitable, at best) steal Dracula's coffin, hoping to replicate his immortality. Dracula is weak, but very much alive. It's demonstrated, for all to see, that his immortality is magic (holy water burns an infected tissue sample). One character deliberately infects himself, another infects someone else to save him (she, at least, was already infected and had yet to turn).
In Judge Dredd, Justice Griffin tries to use Rico to instill chaos and thereby reopen the Janus project, intending to clone an army of Judges to enforce order in the city. He doesn't seem to realize until too late (despite the mountain of evidence) that he has literally no way to control Rico, and gets himself ripped apart by a robot for his trouble.
"Evil is not a toy! It is a highly-collectible deluxe action figure."
At the climax of C. S. Lewis's The Last Battle, Rishda Tarkaan summons the evil god Tash into Narnia. (Oddly enough, he does this unintentionally: he doesn't even believe in Tash, but to support the imposter-Aslan, he claimed that Aslan and Tash are one and the same. Tash hears and decides to pay a visit.) The evil spirit immediately turns on its summoner; fortunately, Aslan (the real one) intervenes before the scene can get any uglier.
Something similar to the above happens in Perelandra. Weston is convinced that God and Satan are merely different aspects of some all-powerful spiritual Force - one which has been teaching him some interesting things. In the midst of a particularly pompous speech, he calls the force into himself, whereupon his will is immediately subsumed by demons. His last words as himself are utterly terrified.
In the novel Guards! Guards!!Lupine Wonse, the Patrician's Evil Chancellor, summons an actual dragon for a carefully groomed hero to 'slay' so the hero can be installed as king, with himself as the brains behind the throne. He still got the position of being the aide of the king ... when the dragon roasted the faux hero and took the crown itself.
Any wizard bright enough to survive for five minutes was also bright enough to realise that if there was any power in demonology, then it lay with the demons. Using it for your own purposes would be like trying to beat mice to death with a rattlesnake.
Likewise, the wannabe witch girls in Lords and Ladies fooled around with elves, despite the old wives' warnings. Disaster ensued, because elves are Not Nice.
An interesting twist in Witches Abroad: Lilith's not-a-toy evil turned on her in the form of her own reflection. Definitely Karmic Death.
In Terry Brooks' The Tangle Box, Horris Kew and Biggar release the Gorse from its prison, and it enslaves them with the intent to send them down the Box when they are no longer useful. In the end, it is Horris who saves the King.
In Robert Weinberg's A Calculated Magic, the Big Bad is Nergal, described as the Babylonian god of disease and decay, who was summoned into the present by the character we meet as his Dragon, who had been trying to summon a demon. Nergal's first reaction was to grip the Dragon's wrist, leaving a set of fingerprint-like spots of pure disease potential, and warns that if the Dragon ever betrays him, that potential will become full-blown...
As big as the Star Wars Expanded Universe is, you know this trope had to come up somewhere. Grand Admiral Thrawn went and fetched the insane clone; but his original was just as bad Jedi Master Joruus C'baoth off the low-tech world he had been stuck on and ruling in order to get C'baoth to use his Battle Meditation and make Thrawn's forces that much more effective. In return, C'baoth wanted Force-Sensitives, specifically Leia's as-yet-unborn kids. C'baoth had delusions of grandeur, and poor Pellaeon kept telling his boss that having any plans involving someone so unstable was a very bad idea. Thrawn did have plans set up for the inevitable betrayal (up to and including growing a new clone as a replacement), at least, but that point where C'baoth took control of a Star Destroyer was... unnerving.
Jerec, in Galaxy of Fear, did not release Spore himself, but when it was semicontained and likely to spread only slowly he decided to strike a deal. He gave it his ship and crew in exchange for it saying that it would obey him - just saying, not even like Thrawn where there was some psychology and safeguards involved. Jerec himself could keep clear of it thanks to The Force.
In Lirael/Abhorsen, it was inevitable that some upstart necromancer was going to try to tap into the power of the Lightning Trap. Anyone who actually managed it got recruited by the Destroyer to help bust him out of his can. Chlorr just ended up as another minion.
In the Warhammer 40000Blood Angels short story Blood Debt, Ramius Stele tries to summon a daemon for study, but makes a mistake and the daemon gets free. Things go to hell very quickly.
Tripos:Because you are more easily put aside than other wizards, of course.
Although as it turned out, Tripos, the Demon King he summoned, was more interested in recovering his enslaved mate from the Big Bad than wreaking havoc in the world. Tripos actually ends up being quite helpful and they couldn't have freed the world from Chesney if Derk had not summoned him.
Draco Malfoy learns this the hard way in Harry Potter and the Half-Blood Prince when he replaces his incarcerated father as a Death Eater, but soon finds himself in way over his head when the Big Bad orders him to perform an almost impossible task (assassinate Dumbledore) under penalty of his entire family being killed.
In the same book, Harry himself learns mysterious pseudonyms are not toys either, no matter how helpful they've been in the past. He tries casting "Sectumsempra" on Malfoy, without knowing what it does, and is horrified to find out it vivisects the target.
In the Biblical book of Genesis, chapter 3, God creates a Tree of the Knowledge of Good and Evil, places it in the garden with His creations, and commands the man and woman not to eat any of the fruit from it lest they die.
In The Fuller Memorandum, later in the series, the main character paraphrases the Lovecraft quote from above when some amateur cultists try to use him as the host for a summoned Eldritch Abomination - and screw it up very badly.
In The Jennifer MorgueBillington intends to fish an ancient continent-crushing war machine created by one of elder races from oceanic depths and repair it because its controlling intelligence mind screwed him into obedience when he tried to contact it.
One of the Robin Jarvis books has a powerful warlock releasing a giant serpent from its magical sleep, believing that he can use his magic to control it. And he's right. When it awakens its will is weak enough that he takes control of it. Then he gets distracted. By the time he can pay attention again its properly awake, and obliterates him.
In the non-canon S.D. Perry Resident Evil novel Underworld, an anti-Umbrella command team get trapped in an Umbrella Weapons testing lab (which lacks a self-destruct.) When they finally escape, they discover a B.O.W. of extreme power frozen in cryostorage, they wake it up so that it can wipe out the facility for them. Averted. It works.
Nakajima of Digital Devil Story learns this lesson very painfully after he summons the demon Loki to kill two of his classmates. After biding his time, Loki turns on Nakajima, and almost succeeds in killing him.
In The Shattered World, the leader of the Circle plots to release the spirit of the Necromancer from its tomb, convinced that the arch-sorcerer who'd shattered the planet centuries ago could be forced to put it back together. The novel's protagonists are Genre Savvy enough to see this trope in the making, and rush to stop this scheme. Subverted, as the revived ghost of the Necromancer lacks the power to reassemble the world, and turns out not to be evil after all; rather, he's the reason why anyone survived the Earth-Shattering Kaboom in the first place.
Anyone who attempted to use The One Ring or a Palantir against The Lord of the Rings learned very harshly that the evil corrupting these items answered to one master and one master only.
In fairness, the palantiri are not inherently evil, being created by the Elves. The trouble is that Sauron has one (specifically the one that was held in Minas Morgul nee Ithil), and can thus reach anyone connected to the entire network. If he wasn't wiretapping them, then there would be no problem.
Randall Flagg of The Dark Tower goes through great lengths in order for Roland's (and the Crimson King's) son Mordred to be born. He is killed later for underestimating what he created.
Merrin Meredith in Septimus Heap fetches the Two-Faced Ring and puts it on so that he can use its powers to kill his arch-enemy Septimus Heap. The ring proceeds to almost squashing off his thumb.
According to Ciaphas Cain, when Chaos cultists seek to attract attention of their patron deity, if they are lucky, they just muck about enacting rituals they don't understand, and start worming their way into positions of influence, where they can disrupt the proper functioning of the Imperium. If they are not lucky, they succeed.
In the backstory of The Seventh Tower, a woman named Kathilde and her brother wanted to depose Mercur, Emperor of the Chosen and take his place. So they released Sharrakor, Sealed Evil in a Can and ancient shapeshifting Evil Overlord, to help them out. Predictably, Sharrakor helped them overthrow Mercur, allowing Kathilde to become Empress and her brother her chief advisor, but in the end they were just walking rubberstamps for whatever Sharrakor wanted done.
Live Action TV
The Master ends up pleading in terror after unsealing the Daemon in the Doctor Who serial The Daemons.
This also happened earlier in The Tomb of the Cybermen:
Klieg: Are you listening? Do you understand me? Now that I have released you— ARGGHH... Let me go! I set you free! It was our plan!
And to Davros, who discovered that he had absolutely no control whatsoever over his Dalek creations. Of course, seeing as how the Doctor had warned him that the Daleks were utterly uncontrollable by anyone not of their race, and how Davros himself had specifically removed all positive emotions (basically, everything besides hatred and self-love) to "ensure they would be the ultimate survivors", this is perhaps more a case of Davros being Too Egotistic/Stupid/Psychotic to Live.
A double dose in The End of Time. In the first case, the Master is brought back by some kind of cult that wanted Mr. Saxon back; then later, he makes the same mistake when he brings back the Time Lords, who want to end time itself.
The X-Files: "Did you really think you could summon up the devil and then expect him to behave?"
In Power Rangers Jungle Fury, Jarrod (accidentally) releases Dai Shi from his box. Dai Shi promptly possesses Jarrod's body. (The box containing the ancient über-evil opened when it was dropped on the ground. You'd think the Pai Zhua masters would lock the box or something, but no.)
Dai Shi keeps releasing more and more overlords, even though in his human shape he's weaker than all of them. The third is finally fed up enough to boot Dai Shi off the throne and take it for himself.
Inverted in Angel when an already free demon possesses a boy, expecting to use him as a host. The boy turns out to be Eviler than Thou, and the demon finds himself trapped inside.
Played straight when Gunn attempts to get help from the conduit in "A Hole in the World":
Conduit: This is the part where I need to be clear. I am not your friend. I am not your flunky. I am your conduit to the senior partners, and they are tired of your insolence. Oh, yeah. They are not here for your convenience.
A room of Wolfram and Hart lawyers discover this after reuniting Darla and Drusilla. Once restored to a vampire (which was part of Wolfram and Hart's plan too) Darla turns out to be rather upset about having been brought back human, dying, and forced to ask for help.
In Cast A Deadly Spell where the villain attempts to release the Great Old Ones into the world. Guess he should have made sure he used a real virgin.
In one episode of Relic Hunter a monk tricks Sydney and Nigel into helping him unleash a demon which promptly kills him. The "evil" abbot who tried to prevent it happening turned out to have been the good guy all along.
The seasonal ChessmasterBig Bads of Heroes regularly fall victim to this in regards to Sylar. They regularly try to recruit him as their Dragon, or at least use him as a pawn in their schemes. After going along for a few episodes, he regularly turns around and makes things end very badly for them. Bennet even lampshades this to Danko: "Just how dumb are you? Who did you think would be left standing the moment Sylar got bored? You?"
In the last season of DS9, Kai Winn joins with Gul Dukat to free the pa wraiths (Bajoran devils) sealed deep inside the planet. While Dukat was insane and knew exactly what he was getting into (hell, they went so far as to empower him), Winn figured she could control both the Pah Wraiths and Dukat. It ended about as well as you would expect.
Earlier in the series, Dukat also thought that he could leverage an alliance with the Dominion into a position of greater galactic influence for the Cardassian Union. In fairness, he was actually managing fairly well until he got captured by The Federation; it was under his incompetent, drunken successor that the alliance turned sour.
A frequent occurrence in Supernatural. The major example is apparently Lucifer, who according to Crowley would have destroyed the demons as soon as he no longer needed them.
The witch who summoned the demon Samhain, and was immediately killed after he had a body.
The idiotic amateur witch who summoned a demon without any kind of protection. It possessed his friend, and offered him "gratitude" instead of the rewards they'd been promised for killing Dean. Then he complained to it about how hard he'd worked and demanded something more, at which point it killed him.
VERY MUCH the case for Castiel at the beginning of Season 7. He sucked in every soul from Purgatory to make himself powerful enough to kill Raphael, declared himself the new God, and lasted a VERY short amount of time (hard to tell exactly in-universe, a few days probably, weeks at most, less than a full episode), and proceeded to get taken over and blown up from inside by the ancient race of evil that had been sealed in Purgatory.
Yapool in the Ultraman Mebius film, Mebius & the Ultra Brothers. Once Alien Nackle (Who was part of a group of aliens with the same goal until the rest were killed by Mebius) succeeded in releasing him... Yapool kills him shortly after, having no need for him anymore.
Seen again the Ultra Galaxy Mega Monster Battle film, when Alien Zarab releases the genuinely EVIL Ultra, Ultraman Belial from a space prison and presents him with a weapon that can control 100 monsters. *Belial however kills Zarab shortly after, not wanting to form an alliance with anyone..
In both series of GARO, the Big Bad falls victim to this trope. Barago is devoured by Messiah after summoning her, and when Sigma Fudou draws power from Ganon's corpse once too often, it triggers a resurrection and he is absorbed.
The meth industry from Breaking Bad: Not only has it turned Walt into a sociopath, but his family is now under constant threat of assassination from the cartels.
Mythology and Religion
The Ars Goetia includes instructions for making sure that conjured demons don't show up in Eldritch Abomination form, but as something more comprehensible...but given the angels described in the book of Ezekiel, this would apply whether or not they had/have a low opinion of humanity. However, there are four exceptions, besides the pomp-and-sycophantry-loving kings (Beleth, Belial, etc.), who get special instructions.
Phenex: Don't listen to his song; in fact, interrupt it as soon as possible (the text doesn't say why, but there's an implication that the conjuror risks becoming Phenex's errand boy...at best).
Valefor: Beware of his temptations to become a robber; he's trying to get you caught and executed.
Malphas: Yes, he accepts sacrifices happily, but that opens the gate for him to start deceiving you.
Andras: Follow the binding procedures to the letter, and ignore every temptation he sends at you to do otherwise. Otherwise? He becomes free to KILL you and every single one of your associates. (No surprise that Andras's powers are causing murder and sowing discord.)
It is a customary rule in all magic-using religions that you do not conjure up something that is stronger than you are. If you don't have the power to send it away again, things can get ugly very quickly.
Which begs the question of why you'd bother in the first place, since if you're powerful enough to control an entity, you most likely don't need its services.
Maybe the summoner has more skill with Summon Magic than with other types. Umineko No Naku Koro Ni has an example of this type with Kinzo- it says in the TIPS that he "can't even use enough magic to reheat tea, but it is possible for him to summon demons that can use enough magic to boil the sea."
Which...still doesn't really make it a good idea, since most once-summoned entities don't just stop existing or develop a case of convenient amnesia once you're done with them. That demon from Fire and Brimstone Hell has been 'safely' banished back home? Wonderful, that means it's now free to plot its revenge where you can't see it anymore...
Another rule is that magic is held by the rules of karma. The more evil the curse is the more likely the spell will backfire. This had lead religions to discourage the use of evil spells in first place.
For that matter, trying to summon any being from the Lower Planes without casting spells to protect yourself and/or contain the creature in question is guaranteed to end badly. A reckless demon summoning will result in both the enemies and the party being killed, if not worse.
In any RPG based on the Cthulhu Mythos, one of the villains will be trying to unleash an Elder God not to end the world, but to harness its power. To date, not one of them has ever succeeded.
The backstory for Warlord villain Avinaar Esmerek looks like one of these, with the heroes finding an ancient temple, obviously recently disturbed, full of prophesies about the awful things that will happen if the being entombed there ever awakens. However, in the final room he is found Killed Off for Real, with a message from the mysterious Avinaar explaining that he unsealed this evil just to have a worthy foe - and it was too weak, anyway. Cue panic.
More than a few denizens of Warhammer 40000 have made the stunningly catastrophic mistake of treating Chaos or something devised by the Necrons like this.
In an especially interesting example, excellent fanfic "The Emperor's Finest"note Not to be confused with the 7th Ciaphas Cain book has, as a plot point, a son of Khorne being summoned and incarnated simply to provide an Ax Crazy daemon prince with a decent fight. The people who try to summon it don't even think about what would happen if it got loose.
A website referenced a Vampire The Masquerade game where evil cultists wanted to revive and enslave Cain, the first vampire, while the PCs were to behold the impending doom, their hands carefully tied. Due to a PC heroically tossing his shoe to one of the cultists, the ritual was screwed up: Cain was revived but not enslaved, and the rest of the world was safe(ish).
This is how one Gehenna scenario ends for the Followers of Set. The clan as a whole descends from an insanely powerful vampire they believe to be the god Set from Egyptian mythology, and worship him by tempting others into wickedness. Well, when he finally wakes from his millennia-long slumber, he can't believe his childer would stoop so low as to worship him, and starts eating them like popcorn.
In GURPS, summoning a demon is temptingly easy to do, even for an inexperienced mage. The probable reason is that demons want to be summoned — by people who can't control them.
In Magic: The Gathering, demon type creatures tend to be powerful for their cost but occasionally have drawbacks that can screw over their controller unless steps are taken to deal with them. Certain demons like the classic Lord of the Pit and Archdemon of Greed demand regular sacrifices from their summoners and attack them if they are denied. Bloodgift Demon and Griselbrand both have abilities that allow their summoners to trade life for knowledge, and players can easily render themselves vulnerable to an opponent's surprise attack. Abyssal Persecutor will decimate your opponent's forces but it also keeps him/her from losing and you from winning just to keep the slaughter going for a little longer. Rakdos the Defiler is so hard to control that he is just as dangerous to his summoner as he is to the opponent.
The Dark Eye has the story of mage-emperor Fran-Horas who, in order to put down a particularly troublesome revolt against his rule, summoned the Archdemons themselves and unleashed them onto the battlefield. They proceeded to wipe out the rebels...then turned on Fran's own forces, plunging the realm into chaos. Fran-Horas himself ended up dragged to hell a few years later, having first had to witness the fall of his empire. And a couple of generations down the line his descendant, the empress Hela-Horas tried to do the same thing. Another century or two of Dark Ages was only averted because the gods themselves decided that enough was enough and made a personal appearance to stop the summoning.
When the Sonic the Hedgehog series got into more complex plots, Dr. Eggman got into the notable habit of doing this.
In Sonic Adventure, Dr. Eggman releases Chaos and feeds it Chaos Emeralds. It obeys him for a while, but eventually turns on him.
Dark Gaia was released from his can (which was the planet, no less) within the first few seconds of Sonic Unleashed, only to break apart and be of no use to anyone, at first. Yet when fully restored, Dark Gaia immediately sends Eggman flying when he tries to control it.
In Sonic Generations, Eggman averts this, as he takes control of a time-erasing creature by mechanizing it and it never turns on him.
Tales of Phantasia: Mars burns down the protagonists' town just to get one of the keys to unseal Dhaos. The first thing Dhaos does when unsealed is to disintegrate Mars. Well, that's the second; the first thing is to reveal that he himself had manipulated Mars to unseal him.
In an early chapter of Shin Megami Tensei II, you are sent to track down the scientist Mekata, and find him opening a gate to the Abyss so he can access an unlimited supply of demon servants. He directs the first demon he gets to kill you... and it immediately kills him instead. Turns out he botched the ritual...
An odd case of this happens in Mortal Kombat: Deception; Big Bad Onaga tricks Shujinko into bringing together the Kamidogu to revive him, then takes them all for himself afterwards in order to fuse all the realms into one. This, in turn, turns out to be another ploy by an even bigger Big Bad, the One Being, who is manipulating Onaga into doing so to revive it, with Onaga's ultimate "reward" being non-existence. And, as a backup plan, the One Being also manipulated Onaga's traitorous successor, Shao Kahn, into conquering realms to fuse with Outworld to achieve the same goal. So far only Ermac, Shao Kahn's former enforcer, seems to be aware of his presence.
The Elder Gods also know about the One Being (they did the sealing, after all), however, they are idiots.
In Gunstar Heroes The 4 Treasure Gems fry General Gray after completing the Boss Rush and reveal the real final boss.
Riku gives himself over to the Darkness at the very beginning of the game, thinking it's an easy way off his world and into the adventure he craves. It ends up separating him from his friends, destroying his homeworld, and apparently devouring the heart of the girl he and his best friend like. Oh, and it also denies him a glorious destiny as the wielder of the Keyblade, but he doesn't learn that until later. He only blunders worse from there on. Having not yet learned his lesson, he makes a deal with Maleficent so he can try and find his friends, and he actively dives into the power of darkness to try and find Kairi's missing heart. When that doesn't work, he makes another deal with some figure he can't even see for more power, which winds up costing him possession of his own body. He finally starts saying "No to Drugs," but it takes him the rest of the first game, all of the next, and most of the third to deal with the consequences.
In addition to all this is Ansem's story (which is really Xenanort's story, as it turns out "Ansem" was really someone else posing as Ansem) in trying to get into Kingdom Hearts, where he plunged headfirst into the Darkness and turned himself into a Heartless. But apparently, he actually needs a physical body to open the door to Kingdom Hearts. Oops. When he finally manages to open the door, the Light within Kingdom Hearts proves to be his undoing. Oops again.
Birth By Sleep reveals that Evil really is not a toy. Use of Darkness be it for good or evil weakens the body's natural Light defenses against other darkness users. Hence trying to fight a stronger darkness user with darkness won't work very well and will just make them hurt you more. Hence Terra who mastered his own power of darkness and swore to wield it against evil and for the sake of his friends (manifested in gameplay by him acquiring a dark Keyblade and an attack called Dark Impulse) uses it against the Big Bad and defeats him with it and then Xenahort unlocks his own heart and sends it into Terra, which Terra whose light defenses are weakened from the use of dark power is unable to resist, causing Xenahort to take control of Terra's body. Evil Is Not a Toy or in this case something that can be used without consequence kids!
While Dark Is Not Evil, Evil is still not a toy, and neither is darkness. It is possible to control the Darkness (Riku finally managed it in his story in game 2) but it is NOT something to take lightly. There's a reason that King Mickey told Riku that losing the power of darkness was a good thing, despite all the past talk about it not being evil.
Vanitas is Ventus's Superpowered Evil Side which was extracted from Ventus's heart and body by Master Xehanort. At the end of Birth By Sleep, he possesses an unlimited army of evil underlings and a nigh omnipotent weapon capable of bending reality and plunging the worlds into neverending war. This is especially distressing because 1) he's a maniac and 2) earlier in the game he showed no remorse over disobeying Master Xehanort's express orders. In all likelihood, he would have just killed his "master" and wreaked havoc all over the universe had Aqua and Ventus not stopped him.
This happens many times in the MMORPGCity of Heroes. In every single case, it's up to the player characters to clean up the mess.
In Paper Mario The Thousand Year Door, the Shadow Queen blows Grodus up after he commands her after freeing her. He thought he could because the Shadow Queen's true servants tricked him.
It also parodies this trope. Throughout the game, the player encounters four black treasure chests with a voice inside them, each of which begs the player to open them and set them free. When freed, the beings in the chest laugh evilly, change the background music to sound more evil, and then gloat about how they have tricked the player and will now curse him. In each case, the "curse" is actually a new ability. This is actually intentional on the part of the "evil" beings, who were the former heroes who sealed the Shadow Queen.
Double Subversion in Wild ARMs and the Alter Code Fremake. The Sealed Evil in a Can, Mother, was being awakened by her Demon children. But once she was released, she told them that she was going to destroy the world, which is at odds with her children's desire to rule the world. So these Demons then betray Mother, helping the heroes to destroy Mother so that it will clear the way for them to rule the world. Mother turns out to be Not Quite Dead; she possesses the leader of the Demons, thus fulfilling the trope.
The arcade game House of the Dead reaches its climax as the Big Bad Doctor Curien unleashes his most powerful creation, Magician Type 0, and commands it to attack the main characters. You can guess what happens to the Doctor.
Subversion, at the end of the first Resident Evil game Albert Wesker tries to unleash the most powerful of the Umbrella Corporation's virus born monsters (the Tyrant) on the main characters. He gets a claw to the torso for his trouble but in the later games it turns out he planned it all along and injected himself with a virus which put him in a deathlike state and gave him superhuman abilities.
Played straight in Resident Evil: Outbreak File #2. In the last level, "End of the Road" a visibly nuts Umbrella researcher releases a Tyrant to destroy the loose hunters, over the objections of the only competent Umbrella employee in the whole series. Guess how it ends.
In Chrono Trigger it's a common belief that Magus attempts to awaken Lavos to play with it before being foiled by the heroes. Subverted; later, if given a chance, he explains that he really wanted to summon Lavos to destroy it. Given the fact that unaltered history went on to having the Mystics lose the war due to Magus' disappearance while Lavos' influence temporarily lessened due to the summoning, it would seem he fought Lavos and only managed to weaken it at the cost of his own life.
Also in Chrono Trigger we have a highly magical floating city of Zeal, whose ruler tries to harness the power of the world-destroying Lavos. As a result the queen goes insane and the city comes crashing down when Lavos gets angry.
Fire Emblem Sacred StonesPrince Lyon of Grado tries to use his country's sacred stone to heal his ailing father. Good idea, except he knows the Demon King's soul is imprisoned inside, but he's sure he can control it. Not only does it control him, but it kills his father, resurrects it as a zombie, and promptly declares war against the world. Oh, and it consumes Lyon's soul by the end of the game. Oops.
Path of Radiance Mad King Ashnard declares war and pisses everyone off so he can generate enough chaos to unlock Lehran's Medallion and free the dark god inside. Luckily you stop him before it happens.
Radiant Dawn Averted. Winds up Lehran is still alive and well. And he starts another war, hoping to break the medallion. He doesn't want the god trapped inside though. But if she wakes up, her counterpart will wake up and turn everyone to stone. And that's exactly what he wants.
Radiant Dawn also retroactively averts Path of Radiance for this trope: the "dark god" imprisoned within the medallion is actually not a dark god at all, merely the embodiment of chaos, emotion, and free will. Which means Ashnard was never using evil like a toy to begin with.
In the World of Warcraft instance of The Arcatraz, the end boss fight consists of a guy releasing monsters to attack the players. In the end he unleashes a big alien/demon/god and gets promptly killed by it.
Similarly, high-level Warlocks can summon creatures which may break free of their bonds and attack the player themselves, though they rarely succeed in killing them.
The warlocks of Auchindoun are particularly bad about this, apparently spending their entire time summoning up things they can't control and desperately trying not to get killed by them. One boss has a room full of people keeping it banished so it can't kill them. And the final boss has a bigger room full of people attacking it and being killed as they try to stop it getting out.
Also, the second boss of Trial of the Crusader is a demon summoned by a Gnome Warlock. He accidentally summons a Eredar Lord, however, who promptly kills him.
Zigzagged with the troll shaman Jin'do the Hexer, one of those serving the Blood GodHakkar the Soulflayer in Zul'Gurub. After his defeat Jin'do's spirit is taken to be tormented by the spiteful god, as one might expect. Less expected is that three expansions later Jin'do the Godbreaker returns to Zul'Gurub and life having enslaved Hakkar. However, his control is still unstable, and if they players manage to break the chains binding Hakkar the god obliterates his treacherous servant immediately.
...Except for in the Demon Path, in which it turns out that the protagonist isn't a very nice person and immediately goes on a genocidal rampage with Gig's powers, starting with her.
Played straight in the normal path anyway, in an unrelated incident. Put short, controlling a World Eater is unfathomably stupid. For the long explanation, King Strauss VIII bought a Crimson Tear with intent to use it to control World Eater Feinne (who is almost completely mindless, so it should tell you how bad it would be to try to control either of the other two). His attempt at fusing with Feinne in an attempt to control her starts with a "hiccup" - Feinne simply blows straight to oblivion the back half of the Kingdom of Raide! He attempts to continue despite this, and, well... OM NOM NOM.
Gig: If you want to know why they're called World Eaters, well... there you go.
Tilt your head and squint, and you can see Bastila and the Jedi Council's efforts to use the greatest weapon - turning someone to their cause, hoping to get the locations of the Star Maps - as this trope. Whether it's averted or not depends on the player.
The Jedi were wise enough to know that this was a dangerous gamble, and would not have done had there been better options.
If you play the Dark Side path in Dark Forces 2: Jedi Knight, Jerec succeeds in his attempt to turn Kyle to the Dark Side...only to have Kyle immediately try to kill him. Now that he's evil, Kyle doesn't have to hold back or worry about innocent bystanders getting in the way. Though Jerec survives that particular attack with no trouble, and ultimately Kyle kills Jerec no matter which path you choose.
In Jedi Academy, one mission starts with the bad guys releasing an unstoppable mutated rancor into a spaceport with the intention of looting the city after its passing. It immediately starts killing and eating them (while they for some reason try to fight back instead of stepping aside and letting it do its thing).
This is the eventual fate of Chancellor Cole in The Legend of Zelda: Spirit Tracks. During the final battle, he and Malladus (the Demon King whom he had earlier released from his seal) seem to make a good enough team together, but toward the end, once their getaway train has been wrecked and the Demon King is forcibly removed from Zelda's body, Cole makes the grave mistake of ordering Malladus to get back into her. Presumably fed up with all the nonsense that had occurred up to that point, Malladus simply eats him, hijacking his body to transform into his final form.
This also happens to Byrne, The Dragon for the Chancellor Cole. He demands Malladus give him power, and it should be no surprise when he gets sent flying across the room and knocked out. It also leads to his Heel Face Turn. Cole even mocks this.
In Dragon Quest VI, about halfway through the game, a kingdom summons a powerful demon named Nokturnus in the hopes of destroying Big Bad Mortamor. The demon immediately obliterates the kingdom.
However, in an interesting twist, if you've done a lot of grinding (and I mean a lot of grinding), you can defeat the demon, who is then so impressed by your strength that he actually does fly over to Mortamor's place and curb stomps him for you. Needless to say, if you're strong enough to beat Nokturnus you're strong enough to curbstomp Mortamor yourself, but it's still fun to watch.
In Final Fantasy V the Bigbad Exdeath has the ultimate goal of gaining power over this thing called "the void" and take over the world with it. He laughs at people saying the Void couldn't be controlled and is seemingly using it without any problems, but after you beat in the game's final dungeon, he loses control of it and possess him, which for you is also bad.
Subverted a bit in Shadow Of The Colossus- At the beginning, Dormin clearly tells Wander that resurrecting Mono will have unpleasant consequences.
During his training, Jubei constantly reminds Ragna that the power of the Azure Grimoire is not truly his power, and that it will never truly be under his control. As the creator of the Azure Grimoire(s), both of them,Yuuki Terumi seems to think he can easily control the power of the Azure. It backfires when Lambda performs her Heroic Sacrifice, enabling Ragna to overwhelm Terumi's Grimoire and halt Mu-12's rampage.
Subverted when Terumi takes advantage of this situation and uploads a virus that freezes the reality warping supercomputer that held his leash. Granted, this did come at the cost of his monopoly over the Azure's power and Mu-12.
This trope also applies to Takamagahara, who paroled Terumi from his Hakumen-enforced imprisonment in the Boundary as a pawn for their plan to destroy the Master Unit. He proceeds to infect them during a blind spot in their omniscience.
Lotte Carmine so believed that if he could harness the knowledge of the Boundary, he could come up with something beneficial to humanity, even if it costs the lives of many people. Even his Mad Scientist mentor Kokonoe did not approve, and the result of that is that he gains nothing, and the Boundary corrupts him into Arakune. Even worse, his girlfriend Litchi decided to dabble on that as well to save him, and on the sign of first symptoms, when Kokonoe refuses to even help her, she decided to dabble with another Evil: NOL and Hazama. It's still unclear which one is Eviler than Thou in this case: Boundary corruption or NOL/Hazama. Lotte at least managed to learn his lesson and managed to pull himself together enough to warn Litchi to stay away from him and get Kokonoe to help her with her own Boundary corruption. Although even Litchi herself realizes that she already dug too far, and just letting it be as everyone else told her to will not make her problem go away. It's complicated.
One of the Six Heroes, Nine, also fell to this. She just banished Terumi after the murder of her would-be brother in law Tomonori. When the Black Beast arrived, she decided to free Terumi and enslave him, thinking she could control him well. That backfired, Terumi killed her and in irony's sake, she would get turned into Phantom, practically Terumi's slave.
In Dragon Age II Merrill knows making a deal with a Pride demon — the most powerful and cunning breed of demon — to restore the Eluvian is dangerous, but thinks the risk is worth it. Nearly everyone around her disagrees, including her mentor Marethari, Anders, Fenris, and Rival!Hawke. In the end her detractors are proven right. The Pride demon was going to use the restored Eluvian to enter the real world and Merrill would have been its first victim. Only Marethari's sacrifice prevented this outcome. Anders is also guilty of this. He believed that becoming Justice's host wouldn't be too bad, since Justice was a "good" spirit. Merrill gently tells him that there is no such thing as a "good" Fade spirit — all spirits are dangerous.
And in Awakening, the expansion to the original game, it is revealed that (weapons-grade spoiler) the Architect, a darkspawn born with free will, accidentally caused the Fifth Blight when he awakened the Old God Urthemiel and tried to turn him into a free-willed darkspawn like himself so that he WOULDN'T start a Blight. Instead, he spread the darkspawn taint to Urthemiel and turned him into the Archdemon. Blight ensued. Oops, sorry!
Shin Megami Tensei has a very simple law of magic-you can attain ludicrous amounts of power by taking demonic (or angelic) essence into your body. Your mind, either because of the power, or because of how you abuse it, is going to degrade to the point you end up doing incredibly stupid stuff and killing yourself through sheer idiocy. Or being enslaved to a divine being for all eternity. Your choice, folks.
Arc The Lad gets a moment of this when the King of a country that has been manipulated by demons willingly sets free the Big Bad and gets as far to say 'My first act shall be...' before he is vaporized for 'Serving his purpose'.
Dead Rising 2: Any Mega Corp that thinks using zombies to further some goal is a good idea is asking for trouble, but the folks behind Terror Is Reality deserve mention for keeping an army of zombies around for a gameshow. Naturally they escape and destroy Fortune City.
The Wish spell in Dominionscan be used to summon any unit you can think of. But if you summon a Doom Horror, it only has a 50% chance of being controllable instead of hostile, and the Horror that responds to your call may not be the one you asked for.
The entire Aurum arc in Kid Icarus: Uprising was revealed to be part of Pyrrhon's plan to take control of the aforementioned Borg expies. It goes as well as you'd think. Notable in that Pyrrhon himself is a god, albeit a minor and not very respected one.
Quest For Glory IV brings us Katrina, a deeply lonely and incredibly powerful vampire and sorceress who desires to unleash a Cthulhuesque Eldritch Abomination called the Dark One, and plunge the world into permanent darkness so vampires can walk abroad at all hours of the day without fear of the sun. It's never really seen what would happen if she had succeeded, as Katrina sacrifices herself to protect the Hero from Ad Avis while the Hero subsequently banishes the Dark One back to where it came from. However based on the Non-Standard Game Over message if the Dark One is released, it probably would not have ended well for anyone.
Homeworld: Cataclysm has The Remnant of the once-powerful Taiidani Empire team up with an extra-galactic viral entity called the Beast. Once the Imperials repair the Nagarrok, the Beast has no more use for the Imperials and starts to devour their ships, adding them to its fleet. Realizing they've been duped, the Imperials flee, leaving the Beast's superweapon defenseless.
In Yu Gi Oh The Duelists Of The Roses, Seto orchestrates the events of the entire game to resurrect a Card Guardian who had struck a deal with his ancestors. Seto expects that the Guardian will help him rule the world. Predictably, the Guardian is NOT HAPPY. And then you find out that Seto has been awakening Card Guardians for years looking for the right one. Dude clearly doesn't learn from his mistakes.
In StarCraft, both the Confederacy and Arcturus Mengsk after them both believed the Zerg were just animals and that it was a good idea to use them as a weapon by putting Psy Emulators that would attract them on any planet they wanted. They were wrong. And they learnt it the hard way.
This is actually invoked in StarCraft II. After curing Sarah Kerrigan from her Viral Transformation in Wings of Liberty, Valerian Mengsk finds out she still has some Zerg mutagen inside her, and asks her to control Zerg cobayes to see how much of her powers remain. Kerrigan quickly grows an army of them, wrecks the entire lab, and then takes them back to their cages to show to Valerian how dangerous it is to think you can control such a chaotic species. Fortunately, Valerian is smart enough to get the message.
UED admiral Gerard Dugalle acknowledged this in his final message to his wife. "The creatures we were sent here to tame are untameable" indeed.
Invoked in Heart of the Swarm: Mengsk has been creating a Hybrid army to use against the Protoss and Zerg. He thinks he can control them; Stukov is less optimistic.
Angel Moxie, Tristan helps release Vashi on the promise that when Vashi's boss Yzin takes over the world Tris can rule France. Vashi lied. Tris got mad and punched Vashi so hard she landed in China, before joining the good guys.
Girl Genius. Zola releases and tries to help The Other. One Gilligan Cut later and she's hog-tied and an unwilling test subject for another fine display of Mad Science.
Vaarsuvius in The Order of the Stick makes a Deal with the Devil and actually does get huge amounts of power temporarily. Having accomplished a couple hits against evil, s/he's now DePowered back to his/her usual self, with the added bonus of putting him/herself in debt, jeopardizing his/her alignment, and making several powerful entities really, really angry. Plus coming back to bite the party in new and interesting ways with the death of the Draketooth clan as well as gods alone know how many other innocents across the Western Continent. Oops.
Xykon also believes (and this is whole plan) that he will be able to control the Snarl. Instead, he'll just unleash a god-destroying monstrosity that has no idea of control at all.Redcloak knows this full well, and has other plans of his own.
In the "K'Z'K" arc of Sluggy Freelance, Gwynn summons a demon to wreak vengeance on Riff. Unfortunately for Gwynn, the demon goes about this by possessing her, stealing her soul, and warping her body into some sort of monster. Oh, and it turns out that, once the demon fulfills its contract, it's free to destroy the world.
In The Gamers Alliance, a greedy mage uses an ancient spell in the Second Age to release the imprisoned demon lord Yurius in order to use him for his bid for power over the kingdom. Yurius immediately kills him for his troubles.
For most of Red vs. Blue: Revelation, the Meta seems to be treated as a guard dog of sorts by Agent Washington, and manages to somehow be a comedic foil for Wash and Doc. At the end of the series, he captures the AI Tex to return his lost powers, and promptly returns to being the Ax CrazyImplacable Man he was in Reconstruction.
Justice League has this with Felix Faust releasing Hades. He learned from his experiences, and was able to live out the more fun side of the trope by possessing Tala later on.
And with Dr. Milo releasing Doomsday.
Milo: And you'll solve both our problems?
Doomsday: Yes. Release me.
Milo(deactivating the restraints): ...Wait! What are you doing?
Subverted in Xiaolin Showdown. The heroes find Jack Spicer trapped in the prison of Hannibal Bean where he tells them that he freed Hannibal and he imprisoned himself there. After getting him out, they find the real Jack outside, and turns out that Hannibal tricked them into releasing him.
Interestingly subverted with the character of Wuya, the spirit of a nigh-omnipotent sorceress who was freed from her puzzle-box prison by Jack in the pilot, and promised him the world if he helped her. Although Jack never quite succeeded and eventually became her adversary, when Raimundo restores her powers with his Face Heel Turn, she repays him by... granting his every wish and grooming him to rule the world alongside her. It seems she was a villainess of her word after all.
In "To Save Skeletor", Skeletor decides to summon an elder god. You can guess from the episode title how that worked out.
In "Computron Lives", an episode of The Galaxy Trio, a group of would-be terrorists find the deactivated Computron, the first villain of the series, and decide to wake him up so they can use his power to conquer the world. They survive the experience, but really ought to have known better, given that his defining qualities had been his self-determination and contempt for organic life.
In Teen Titans, Slade cuts a deal with the demonic Trigon in order to get raised from the dead, in return for Slade helping the demon take over the world. After Slade fulfills his role, Trigon turns on him. Subverted in that Slade knew exactly what was going to happen and took precautions. He ends up orchestrating Trigon's downfall and getting his mortal life back, and is still at large at the end of the series while Trigon either got completely destroyed or re-sealed in Hell.
Technically, they were sealed in the same can — Trigon was in Hell because he was a demon, and Slade was there because he was evil and dead. However, Trigon seems to have drawn Slade's soul to him, and he was the one who instigated the deal. Slade took it because it was the only chance he had.
In an episode of Darkwing Duck, F.O.W.L. decides to find Taurus Bullba (the villain from the pilot and the only non-goofy villain of the series) and rebuild him as a cyborg. He's grateful... just not grateful enough to serve them.
Actually, he seemed rather pissed about being brought back as a cyborg. His voice is dripping with bitter sarcasm when he says, "So what if I have to drink motor-oil cocktails for the rest of my LIFE?"
In the comics, F.O.W.L. high command decided to revive Duckthulu. Surprisingly, Duckthulu thanked them by disposing them and continuing to destroy all of reality.
Dexters Laboratory: A rare subversion occurred in one episode, "Jeepers, Creepers, Where is Peepers?", where GOOD is not a toy. An evil alien warlord kidnaps DeeDee's first imaginary friend, Peepers, a cute yet obnoxious furry critter who rules over her imaginary world of Koosland, to use as a potential energy source for his secret weapon. After being battered around by Dexter and DeeDee's other imaginary friend, Koosy, the villain's taunting is cut short as he realizes in an Oh Crap moment that he is about to be smushed into a pancake by Peepers, who has mutated (not transformed, but mutated) himself into a flesh-coloured, evil-looking dragon (who also makes his own evil-sounding laugh afterward).
In the five-part Centurions episode "Man or Machine", Mad Scientist Doc Terror recreates an alien Master Computer on Earth because they share the same goal—ridding the world of organic lifeforms. Terror assumes that as a cyborg, he'll be spared. He's wrong.
Shredder from the Teenage Mutant Ninja Turtles 1987 series has problems with this. In the pilot, he teams up with the brain from Dimension X Krang, but refuses to create a new body for him because of this trope, but caves in later on. Once the bad guys and the Technodrome end up in Dimension X, Krang takes charge. This pops up later on in Turtles Forever, when Shredder rescues his counterpart from the 2003 series from imprisonment on an asteroid for the purpose of a Villain Team Up. Unfortunately, the Darker and Edgier Shredder not only takes control of the Technodrome and Krang's technology, but also decides to wipe out creation himself.
In Transformers Prime, Megatron tries to revive Unicron so he can rule by his side. When Unicron wakes up he tells him he was wrong about two things: 1) Unicron woke up himself without Megatron's help, and 2) Unicron wants to destroy the universe and everything in it, including the Decepticons.
In "Birds of a Feather", an episode of Batman The Animated Series, Veronica Vreeland brings the Penguin into her social circle as a publicity stunt. He eventually finds out he's being used, and in his true flamboyantly villainous fashion, kidnaps and attempts to murder her.
In Scooby Doo Mystery Incorporated, Professor Pericles spent the entire series and a good chunk of its backstory trying to free the entity in the sarcophagus that drove most of the plot hoping to claim its power. In the end, he finally does, only for said entity to use Pericles as a physical vessel, killing him in the process.