Magic Misfire

A character has magical powers. Either because of inexperience, a Malfunction Malady, the magic equivalent of Phlebotinum Breakdown, or some other reason, the spell screws up, often resulting in an unintended effect or Amusing Injuries, or both. In a game, a Magic Misfire can be a subtype of Critical Failure. Derived from D&D's "Magic Missile", it's stock in trade for the Inept Mage.

Anime

 * Pretty much anything Skuld from Ah My Goddess tries to do with explicit magic, though she fares a little better with quasi-mechanical magic devices.
 * Same goes for Yuuma from Magical Pokaan.
 * In Scrapped Princess, Raquel's attempt to invoke her powerful Thor lightning spell (after she had thoroughly smashed a Mauser priest earlier) ends with the church in ruins. Shannon reminds her that she has a tendency to overdo it, especially when doing simple tasks, like cooking.
 * Negi Springfield of Mahou Sensei Negima had a bad habit of accidentally invoking wind magic whenever he sneezed. This normally resulted in a sudden gust of wind (always around girls wearing skirts, for some reason), or a clothes-shredding disarming spell. Once, however, it helped him win a Beam-O-War.
 * Mana on Yu-Gi-Oh -- she got better.
 * Chacha in Akazukin Chacha.
 * In what has to be the strangest example, in Happiness, two boys hit with a combination of attack, defense, and butterfly spells are turned into magically sexy girls until expending a one-time magic charge.
 * Actually, Magic Misfires happen all the time in Happiness!. Especially when that magic-user tries any Mundane Utility.
 * In Magic Users Club, Sae Sawanoguchi's immense (but poorly-controlled) magical power means that the net effect of any magic the club performs as a group is a result of her passing stray thoughts.
 * In Di Gi Charat, Puchiko's attempts to use her Eye Beams power cause her eyes to emit strange slime creatures that crawl away.
 * A common problem when attempting more powerful kidou in Bleach, Renji in particular seems to have every other kidou he cast explode in his face.
 * Free of Soul Eater has been in prison for a long time, and is thus out of practice when it comes to fighting, so this happens to him several times. Good thing he's immortal.
 * In Yu Yu Hakusho Rando has has acquired the techniques of defeated martial arts masters and used a shrinking curse to defeat Kuwabara. He tries the same thing on Yusuke but it backfires. Genkai explains that a curse will work on the user if the victim cannot hear it (at that moment Yusuke had algae in his ears). Yusuke is too tired to continue fighting at that point, so he lets gravity finish the job.

Comic Books

 * Runaways has Nico and the Staff of One, which has the limitation that you can only use the verbal command for each spell once; trying it again will result in something entirely different, such as when she repeats herself in battle and ends up teleporting with a team mate out into the desert.

Fan Works

 * Everywhere in My Little Pony Friendship Is Magic fanfiction. Twilight messing up a spell has caused body swaps, induced time travel, sent her friends on long-distance teleportation trip, grafted Rainbow Dash's wings to Applejack, hauled in multiple creatures from other worlds (most frequently humans), and blasted Equestria into its component atoms, and that's just the shipping fics.
 * In With Strings Attached, As'taris's ineptness with a stone-to-flesh spell is apparently what causes Paul's transformation. It's actually the C'hovite gods empowering him.

Film

 * Whenever Ergo the Magnificent from Krull tries to cast Baleful Polymorph, he ends up hitting himself (explained as Hillfolk wizards "laking the power to do real harm"). He eventually learns how to use this to his advantage.

Literature

 * The Harpels of the Forgotten Realms books.
 * Eh, I always got the impression that it was less a matter of miscasts and more a matter of them being Magical Mad Scientists who researched utterly bizarre spells and frequently used said spells before they figured out the side effects.
 * It's a little of both, some of the spells are bizarre, but function more or less as intended. Then there are the times this happens.
 * Schmendrick the Magician from The Last Unicorn.
 * Fizban in Dragonlance.
 * The wizard Ebenezum in A Malady of Magicks and all the following books was seeking a cure for his allergy to magic. He could get very creative about avoiding spellcasting, which would make him sneeze fit to blow his head off.
 * And of course, The Sorcerer's Apprentice.
 * In Discworld pretty much all magic works like this: With the amount of difficulty involved in getting magic to do what you want, as well as its side-effects and leakage, it's treated a lot like nuclear power; it's extremely powerful but is only used by a select few, because the average person would rather just use a bonfire and be done with it than risk using a bar of plutonium to cook a fish.
 * Ron Weasley has neither the raw determination of Harry or the bookish scholarliness of Hermione, and so can seem to lag behind the other two in magical power or skill, but nothing was worse for him than when his wand was broken. Offensive spells tended to rebound on the caster--i.e. him. This, however, turned out to be a good thing when Gilderoy Lockhart stole Ron's wand and tried to erase Harry and Ron's memories with it. (You can guess what happened.)
 * This happens all the time to Questor Thews in Terry Brooks' Landover books.
 * Happens constantly to Wodehed, the Inept Mage of the Welkin Weasels. He's turned wineskins into frogs by putting potions in them, leaving the drinker unharmed, attempted to turn a shrew into a human and technically succeeded but the result was still only an inch tall, and caused a rain of apples when attempting to conjure a bottle of juice.
 * In The Magicians, casting advanced spells when upset will generally result in the caster transforming into a creature of pure magic, with lethal results (both for the caster and for anyone in the way). As a last resort in the battle against,   does this deliberately.
 * There is a children's book once about a boy who found a magic book, and learned to juggle out of it and do magic. Eventually he is called upon to stop a dragon and save the kingdom, but he can't use any of the spells correctly. The first spell, to create a wall makes a giant paper fan. Against a Dragon. The second spell makes an army of Snowmen. Again, Fire-Breathing Dragon. The third spell . It Makes Sense in Context. What was the name of that story...?
 * The villains of Kitty Takes a Holiday- or at least the first part- try to lay a curse on Kitty's cabin as part of a Scooby Doo Hoax to get rid of her. Not only does it not work due to their inexperience, but  Curses are not toys, people.

Live Action TV

 * Every single episode of Wizards of Waverly Place.
 * When Piper Halliwell's powers evolved from freezing things to full molecular manipulation, her misfires made things explode.
 * Aunt Clara on Bewitched.
 * Willow, several times, on Buffy the Vampire Slayer. Once, she accidentally lets loose her vampire doppelganger in Sunnydale, and another time, her argument with Anya lets loose Olaf the troll.
 * Similarly, Angel had Lorne's spell to restore Cordelia's memory, which backfired and turned everyone into their 17yr old selves.

Tabletop Games

 * Havoc Paradox in Mage: The Awakening causes this to happen to spells, at best causing them to hit an unintended target, at worst rearranging the spell's effect. Given that Paradox is 'the manifestation in this reality of a truly abhorrent flaw in the universe this is always, always a bad thing.
 * And Havoc's just one form of Paradox. For instance, there's Branding, which warps your appearance to take on mythic (and monstrous) elements associated with your Vice. And at the top of the clusterfuck chain is Manifestation Paradox, where one of those things from the aforementioned flaw in the universe enters our world and starts rearranging the furniture.
 * Warhammer 40000 has this as a rule for psychic powers, with a chance of a "Perils of the Warp" attack rolling a double 6 or double 1 on a Psychic test. The psyker suffering from the Perils will take a wound with no saves allowed (except invulnerable save, but it has to be rerolled if successful). On double 1 the power still works.
 * There are other side effects.
 * Warhammer has an extremely similar rule, Miscast. Here a roll on a chart is used to determine the effect of the miscast, ranging from the spell just failing to the mage getting sucked into the Warp.
 * Orc and goblin shamans draw the energy from other goblinoids and consequently aren't vulnerable to the Warp in that fashion. But if a orc loses control of their magic, the extra energy can make their heads blow up.
 * The roleplaying game variants of both the above go into gloriously horrific detail about just how badly one can botch a roll. Best case? Milk around you spoils, and unnatural winds kick up. Worst case? Chaos/The Warp eats you.
 * GURPS uses a similar system to Warhammer. The effects range from nothing happening all the way up to a malevolent entity of near absolute power being called into existence with the sole intention of making your life difficult. Alternate critical tables offer anything from funny to summoning things that are even worse.
 * The "Wild Mage" class in Dungeons and Dragons, best known for appearing in Baldurs Gate 2, casts the same spells as a standard mage with a 95% chance of expected results . . . and a 5% chance of something else, ranging from dropping a cow on the target to summoning gemstones out of nowhere. For added fun, their unique level 1 spell "Nahal's Reckless Dweomer" can mimic almost any other spell but has a much higher chance of bizarre effects.
 * In some editions, arcane spells also have a chance to simply fail outright if the caster is wearing armour. Deafness and some other conditions can also cause a chance of spell failure, as can being distracted during casting (by taking damage, for instance).
 * For just about any RPG that uses magic, it's considered traditional to have a long and varied list of the various misfires that can occur when a spellcaster rolls poorly. Hilarity Ensues.

Theater

 * Wicked. Monkees that fly, Steal a man's heart, Walk again, oh my. And then there's  a (preemptive!) Came Back Wrong.

Video Games

 * Happens in the Wizardry series-all spellcasters barring those who use the alchemy spellbook have a skill called Oratory, representing the character's ability to correctly speak magical incantations. Until this skill is built up, spells have a chance of fizzling or backfiring on the party, and errant backfires have destroyed many a party. Alchemists and those who use their spellbook (rangers and ninjas) are fortunately exempt from this, but if they do change to another type of spellcaster, they have to then learn Oratory or their spells will fizzle and backfire.
 * In Saga Frontier, the spell "Psychic Prison" forces one of these on the target--The next time they try to use magic, the effect "Backfire" occurs instead, damaging them.
 * In Final Fantasy II, low-level magic has a higher chance of not working, except for Fire, Ice, Lightning and some other Black Magic, and Cure, which simply do less damage / heal less. Some spells have nasty side effects at low levels; Teleport, for instance, will send you outside the dungeon when cast out of battle, but it drains the caster to between 1% and 0.3% of their max Hit Points.
 * In certain Angband variants (like Zangband), Chaos mages can cause chaotic side effects when they fail a spell. Usually they're not too bad, but they can be pretty awful. Or awesome.
 * The backstory behind Rikku's fear of lightning is that her brother once tried to use a lightning spell to protect her from a monster.....and hit her instead. Ouch.
 * In Dungeon Crawl every spell has a chance to misfire, with the consequences growing progressively more catastrophic with the strength of the spell and the extent to which you failed. Botch a routine ice dart and you might get some frost on you, flub a complex translocation spell you aren't qualified to use and you might find that you aren't in Kansas anymore. If that wasn't bad enough these failures cause magical "glow" that builds up in your system. Glow causes ill effects such as mutations and inevitably culminates in a violent terminus for those too desperate or stupid to stop casting.
 * In Ultima Underworld 2, your character can damage themselves with their own spells if their casting skill isn't high enough.
 * The weakest Imps in the Dragon Quest series, and especially Rocket Slime, fight almost exclusively with these. They're really terrible at magic.
 * In Rocket Slime, standing still while an Imp tries to attack you will cause his magic to explode in his face. Running will cause him to chase you, trip, and explode, damaging everything in the area around it, meaning you, other enemies, and itself. This is all they can do.

Web Comics

 * Black Mage from Eight Bit Theater once missed a volcano.
 * In Sluggy Freelance pretty much every spell cast using the Book of E-Ville invokes this trope.
 * Dan and Mabs Furry Adventures has refused to learn or use magic ever since he accidentally hit himself in the face with a rock as a child.
 * Ratheel of Juathuur learns the power of words.

Web Original

 * In the Whateley Universe, the mage Fey is powerful enough that she routinely generates hobgoblins by accident. Her teammates have learned to look out if they hear her say 'Oops'. The first time she had PMS, there were real thunderstorms in the dorm hallways. And monsoons.
 * This tends to happen whenever Perf of Journey Quest tries to cast a spell straight from the book rather than from memory.

Western Animation

 * Gwen was a frequent victim of this, before she gets her Pink Lantern powers in Alien Force.
 * One episode of Darkwing Duck had Morgana try to use magic to fix the main character's hair, but she accidentally turned him into a yak.
 * Anna Maht's spells in World of Quest.
 * Due to a slip of the tongue in Fern Gully, Crysta shrinks Zack down to fairy size, as opposed to giving him the gift of fairy sight, but at least he got an important lesson about the environment out of it. Nearly got him killed in the process though.
 * Happens in Jackie Chan Adventures when Jade, who shouldn't be playing with magic in the first place, casts a spell and something besides the desired result occurs. This is done typically by casting the spell incorrectly or misunderstanding the spell altogether.
 * Presto from Dungeons and Dragons. It got old pretty fast.
 * Usually what happened turned out to be useful in some manner he hadn't expected, such as a garbage can lid shield giving courage to a coward.
 * Orko from He-Man and The Masters of The Universe is this trope incarnate. His whole gimmick is that he was actually a great magician from another world who lost his ability to cast spells properly due to losing a certain magical artifact (which was either a medal or a wand depending on which version you watched). Because of that he always ends up shooting flowers out of thin air whenever he tries to summon a tornado or something. Like Presto above, his shtick got old pretty fast as well. Why there isn't a picture of him messing up a spell on top of this page is a mystery.
 * Happens repeatedly through Princess Gwenevere and The Jewel Riders. Often to the evil Lady Kale, unable to fully control her Dark Stone (once she even ruined her own castle while trying to blast the Jewel Riders and then made her dragon fall on top of her) as well as the Crown Jewels she captures, ending especially badly (for her) when she manages to steal them all. In "The Faery Princess", all of the magic in the Faeryland was working in a different way.
 * Happens a fair bit in Trollz, as spells can vary depending on how you word them.