Ox blood, the tail of a scorpion, head of a hydra, and an orphan. Sorry, that orphan appears to be taken for another ritual. Will a Wholesome Crossdresser do as a virgin sacrifice?
A ritual goes fine, except a key ingredient is left out or switched at the last minute. There are usually two reasons for this. The first is that if the ingredient comes from The Hero, it allows them to still be involved in the fight. The second is that the creature created that otherwise would have been invincible, now has a Weaksauce Weakness or Achilles' Heel, inherent in the faulty ingredient.
Compare and contrast Gone Horribly Wrong and Gone Horribly Right, for specific experiments with disastrous consequences.
Anime / Manga
Naruto has a ritual involving all the eye bloodlines, and all the tailed beasts. Tobi can't capture the two strongest tailed beasts, so he just uses small samples of their power. It appears to have worked for now.
In the original Magical Girl Lyrical Nanoha, Precia Testarossa needed all 21 Jewel Seeds to open a portal to Al-Hazard, but only ended up in possession of 9, "thanks" to the title character and the Bureau's efforts. In the end of the series, she decides to go through with the incomplete ritual and goes MIA. Her ultimate fate is still a major mystery of the series.
Fox Legend has a ritual to awaken the Sky Demon which involves the Wei family seal, and the heart of the last member of the Wei family. The heart is substituted, though.
Another "sacrifice is not a virgin" example: in Jennifer's Body, the rockers sacrifice Jennifer thinking she's a virgin, but since she's not, a succubus takes over her body.
In the German comedy film Die Nacht der Lebenden Loser (Night of the living losers, duh!), a trio of wannabe satanists try to summon the power of some kind of evil spirit... Unfortunately, they lack the proper ingredients and have to make do with what's available to suburban teenagers: A slightly bloody band aid instead of proper human blood, a full ashtray isntead of a dead murderer's ashes, and a frozen chicken from the supermarket (!) instead of a black rooster. The spell works anyway, although not as intended.
Subverted, as usual, in Discworld: wizard magic is often done with an elaborate ritual, but most of that is just for looks. For instance, the Rite of Ashk'Ente, only needs one wizard, three bits of wood, and a fresh egg. If you haven't got a fresh egg, a mouse will do. But wizards generally feel that if don't have eight archmages chanting at the corners of an octagram filled with occult paraphenalia, you aren't doing it properly.[[note]]Description ganked from Hermetic Magic. Thanks![[/note] Witches are more practical; they're not above doing something impressive for headological purposes but when nobody's watching will take whatever shortcuts are available.
Inverted in Guards! Guards!: the Elucidated Brethren of the Ebon Night are trying to summon a dragon, and the cult leader orders the brethren to find magical objects to sacrifice. They come up with really low grade magical junk, like a still-fizzing letter from a bar and an amulet the Snake Oil Salesman swore was magical. And yet, it works.
Russ Martin's novel The Education of Jennifer Parrish. A coed is chosen to be the victim of a Satanic cult ritual in which her mind will be switched with the mind of one of the cult's leaders so the leader can gain a young body. The ritual proceeds, but ultimately fails because the coed isn't a virgin, causing the death of the cult leader.
In Doctor Who "The Armageddon Factor," the last serial in the "Key to Time" Story Arc, the Doctor and Romana I have five of the six pieces of the eponymous key and make a makeshift sixth piece since they know what its shape is. It works, but not well.
Babylon 5: in the episode "By Any Means Necessary", Londo buys the last rare G'Quan Eth plant for sale on Babylon 5 before G'Kar can. G'Kar needs it for an important religious ceremony that must be held annually when the Narn sun falls directly behind a certain mountain on the Narn homeworld. Londo eventually lets G'Kar have the plant (after using it for recreational drug purposes), supposedly too late for the ceremony—but Sinclair convinces G'Kar that since his home sun's light continues to travel through space, and Babylon 5 lay almost exactly 10 Narn light-years from Narn, that the light that hit the mountain in the proper position 10 years ago would be the same light that would just now be hitting Babylon 5, so the ceremony could still theoretically go on at the station.
Kolchak: The Night Stalker episode "The Youth Killer". Helen of Troy (yes, the one from Greek Mythology) has retained her youth down to the present day through the Human Sacrifice of perfect physical specimens to the goddess Hecate. Kolchak points out to Hecate that one of Helen's recent sacrifices had a glass eye, causing Hecate to withdraw Helen's stolen youth and Rapid Aging her to death.
One episode of Buffy the Vampire Slayer has Willow using the eye of a frog dissected in science class in a potion that calls for eye of newt. Apparently "eye of amphibian" was good enough, because it works just fine.
In Kamen Rider Wizard, White Wizard with the intention to recreate the Sabbath again to revive Koyomi, hijacking Kamen Rider Wizard and the 3 others Kamen Rider Mages and immobilized in four points around the city, but the ritual cost many lives however Nitou (Kamen Rider Beast) appears to stop him, he lose the fight, but use the Hamel Cane to destroy your drive releasing Beast Chimera and eating all Phantoms from human soul, and cancelling the Sabbath.
Dungeons & Dragons. In D&D, improperly marking part of a summoning diagram allowed any creature summoned by that diagram to attack its summoner. An old piece of Dragon magazine art had a demon holding a magic-user in a Neck Lift over the diagram used to summon it. The demon is pointing to the diagram and saying "Missed a spot."
In Age of Aquarius, the game mechanics allow you to try this at your own risk, with a penalty called magic dysharmony. Some components of rituals are essential and can't be swapped, however.
The Elder Scrolls IV Oblivion: In one questline, Umbacano, a collector of Ayleid artefacts, asks you to aquire an Ayleid Crown of Nenelata that is in the possession of a rival collector; the second collector refuses to sell, but suggests that an Ayleid Crown from Lindai might be similar enough to fool Umbacano. Whichever crown you give him, Umbacano reveals that he wants to use it as a component in a ritual. If he has the Nenelata crown, the ritual will turn him into a lich, but if he has the Lindai crown, the ritual will kill him.
In The Elder Scrolls V: Skyrim, a mage at the College of Winterhold tries to duplicate the ritual that either caused the Dwemer to Ascend to a Higher Plane of Existence or collectively die, depending on whom you ask. Luckily for humanity, his substituting a modified soul gem for the Heart of Lorkhan means the ritual only kills him.
Devil May Cry2: Subverted. Dante attempts to sabotage the ritual to summon the Demon Prince Argosax by replacing the Arcana Medaglia with an ordinary coin, but the spell still creates a portal to the demon world.
The Legend of Zelda Oracle games: The witch sisters Twinrova are preparing a ritual that will resurrect Ganon, by sacrificing Princess Zelda, but Link interrupts the ritual. They are forced to sacrifice themselves in order to finish the ritual, which creates a mindless, raging Ganon that Link is able to defeat.
Sacred: Shaddar attempts to summon a sakkara demon and bring it under his control, but one of his minions messes up while drawing the pentagram on the floor. As a result, the ritual summons a sakkara which is obviously not under Shaddar's control, and rather annoyed at being removed from its natural habitat...
Shin Megami Tensei: Nocturne: At one point you meet a Baphomet who is preparing a ritual to summon the demon Mara, and a pair of manekins who are pestering him to hurry up. If you side with the manekins he'll do a quickened version of the summoning ritual, causing Mara to take the form of a Blob Monster instead of his true form.
In Sa Ga 2, Apollo assembles the MAGI to become all powerful, but doesn't realize that he is missing one of them. Because of this, his One-Winged Angel form is flawed and the heroes are able to defeat him.
In Disgaea 2: Cursed Memories, to summon "the strongest demon", Adell needs the fingernail of an Overlord. He asks Etna (who previously slaughtered him and his party in a Hopeless Boss Fight) for one of hers. She gives him a fake nail, because she's a demon and therefore a Jerkass. As a result, the ritual summons Etna herself, reduced to level 1.
In The Curse of Monkey Island, the solution to a puzzle near the end is for Guybrush to recreate a magic potion from earlier in the game, using creative substitutions for some of the ingredients that aren't available in the place where he's now trapped. It still works.
In BIBLE some evil cultists use an old summoning circle to summon the Great Sin Wrath. However, this circle was damaged, causing several cultists to be disintegrated by the ritual's power and later making it easy for Balthazar's angel troops to destroy the circle.
Jackie Chan Adventures: Tohru whips up a potion to prevent Draco from absorbing demon powers, but the last ingredient must be a piece of Draco himself. Jade substitutes a piece of Draco's father Shendu instead. But we never get to see if it works, because Draco manages to go One-Winged Angel too quickly.
In the Teenage Mutant Ninja Turtles (1987) cartoon, Shredder is creating an incantation to access Dimension X using magic, as opposed to the usual dimensional transport tech that he and Krang have access to. However, the ritual has to be performed exactly as transcribed or there will be terrible, unmentioned consequences. Shredder manages to stay one step ahead of his foes, and almost complete the ritual, however at the very last word of the ritual, Splinter bursts in and yells "STOP". This causes the magic portal to open to Dimension ''Stop'' as opposed to X, where an Eldritch Abomination resides. Said Eldritch Abomination tries to escape and kill everyone, good or bad, and chaos ensues.
In 1st generation My Little Pony movie, the witches create Smooze, a giant mass of sentient, evil slime. It is supposed to be undefeatable but the two younger witches didn't bring floom ( blood/sap from a triffid expy - they were too afraid to go get it) and hide the fact from their mother. This weakens the Smooze enough for the rainbow to stop it the first time round, allowing the heroes enough time to go on their quest to find help.
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