"However, the giants of the Sul’at League studied the Gyrderi and found a way to counter the druids—the giants enacted a terrible curse that forever bound them in the wild shapes they were wearing, trapping them and their descendants in the forms of animals."
You know how, when you were a kid, your mother always said that if you kept making that face, it'd stay that way?
This is the Shapeshifting equivalent, where the shapeshifter gets stuck in a particular form, unable to use their shapeshifting powers. If they're lucky, they're stuck in their most natural or inconspicuous form (possibly their Shapeshifter Default Form), but if not, the body they're stuck in is either weak and pitiful, or monstrous and riot-inciting.
Conversely, milder forms of Mode Lock may lock someone out of a certain form instead of in to one, or require that they stay in the form they take for a certain amount of time. In practice, used to establish contrivance to why a shapeshifter may not look optimal to a situation.
The name is taken from Transformers, more specifically, from the Transformers: Universe profile for Cloudraker, where it is in an issue of the Marvel comic book, which states "His major physical problem is a tendency to suffer "mode-lock" — the inability to transform from one mode to another."
Compare The Mind Is a Plaything of the Body, And I Must Scream, First Law of Gender Bending. See also Power Incontinence. Contrast Involuntary Shapeshifting.
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Anime & Manga
The goal of the cursed characters in Ranma ½ was to be cursed again in the Spring of the Drowned Man or Drowned Girl. That way, when splashed with cold water, they'll turn into themselves (effectively not changing). Aside from that, the more common disadvantageous type of Mode Lock appears a few times as well:
In an arc in the manga we find out someone with the same curse as Ranma has been locked into cursed form by the water from a magical pail and is looking for the cure. Later, when Ranma pisses him/her off in a fight, he uses it on Ranma. Mousse and Ryoga go with Ranma planning to use it on themselves while in human form to effectively remove the curse, but since it works by splashing you with cold water, you'll always be in cursed form when the Mode Lock take effect.
Ranma also briefly had a temporary cure when he got a fever so hot it made any water he came in contact with too warm to activate the curse. Conversely, when he got a freezing cold later in the same story, the moisture in the air flash-froze in contact with him, which also made it impossible to turn into a girl (since solid ice doesn't trigger the curse).
Don't forget the second movie Nihao My Concubine, in which the villain has waters from a sacred spring that transformed the one splashed in it permanently into a man. He uses it to threaten girls to come along quietly with him, but the curse spring victims try to use it to break their curse. By the end of the movie, Ranma's falling high speed toward the spring, but he has Akane falling with him, so he destroys the spring so she doesn't get stuck as a man, ruining his chance to break the curse once again.
Said Mission Control, Ryou, who himself has a cat form named Alto is specifically said to be able to stay in cat form for no more than ten minutes - or else the transformation will become permanent.
In the second season of Corrector Yui, the corrector Synchro, who had spent almost the whole first season trapped in the form of Corruptor War Wolf, was forcibly reverted to his War Wolf shape when he was attacked and infected by a powerful virus.
In Claymore the more a Claymore uses her Youki power, the more her body transforms into a Youma — the very thing they hunt (since they are part Human part Youma). If they use up 80% of their Youki power, they fully turn into a Youma and cannot transform back into human form and hunter will become the hunted (apart from some rare exceptions).
Keel's goal of undoing his human transformation in Buster Keel. He does manage to gain a few items that let him temporarily transform his arms to their original forms for fighting.
Happens twice in Those Who Hunt Elves - once involving a shape-change spell, once involving a lycanthrope. In both cases, having a spell segment imprinted on one's body creates a Mode Lock as long as the segment is in place.
An example from Otogi Juushi Akazukin: Valused to be a werewolf. However, thanks to injuries he sustained while protecting Akazukin's home village from other werewolves several years before the series began, he's now trapped in wolf form.
Digimon Tamers was the first series to explore this aspect of E[Digi]volution — both Guilmon and Terriermon have some trouble in coming back to their Child/Rookie forms after the change; there was even an entire episode showing the problems in hiding the huge Growmon in the real world. Eventually this aspect was downplayed, since they spent some 20 episodes in the Digital World, and after that the Digimon's existence was known by the public.
Marvelous Melmo has two chief examples of this trope. Melmo, the titular heroine, is gifted from her dead mother of two types of candies. The blue candies make her older, and the blue ones make her younger, with the explicit purpose of making her able to care for her younger siblings. As early as the first episodes, Melmo discovers that by taking both pills at once, she can de-age herself to embryonic state, then regrow her body in another form, thus shapeshifting to another animal.
When his little brother Totoo tries that for himself, he turns himself into a frog, spending half the series figuring how to swallow a couple of pills that now are bigger than his own stomach
When the angels responsible for Melmo's empowering decide that she's abusing it, by continuosly aging and shapeshifting for personal reasons, they withold their blessing from the candies. However, at the very same moment, Melmo is currently transformed into a dog, so, without her candies, she's stuck in that form, and unable to care for her little brothers. Upon getting her Aesop, she's turned back, but this time with a limited supply of candies.
Akko-chan, from Himitsu No Akko Chan usually had to use the mirror to switch back as well, causing problems when she would temporarily misplace it.
In one episode of the original 1969 series, aptly named _____, Akko-chan, upon meeting a new deaf kid, uses her mirror, out of curiosity and compassion, to transform herself in a deaf-mute version of herself. Too bad that, since the mirror works by clear utterances of the needed transformation, and since Akko-chan insisted on the "mute" part of her ailment, Akko-chan couldn't ask the mirror to be changed back anymore.
Apparently, the mirror could have hit the Reset Button all the times, but but since it believed Akko-chan's desire for deafness shallow and impulsive, had decided to show her how serious is an handicap by threatening Akko-chan with a permanent mode-lock, with the added perk to show how badly she was missing the point: instead of trying to feel on a temporary basis what deafness is, she should have rather stopped at how her new friend was going on even knowing that he couldn't get a magic mirror to fix him.
Happens to Moka in Rosario + Vampire when she is unable to reattach her rosary and return to her "outer" personality. It takes more than a dozen chapters for this to get fixed, and it's still hinted that the two personalities are slowly becoming one.
In SD Gundam Force, Captain is stuck in vehicle mode after his soul drive is stolen.
This happens to the Macross in The Movie of Super Dimension Fortress Macross: when a Meltrandi attack blows off its Eva Fins, the Macross is locked into Humongous Mecha mode since those fins, aside from being its Wave Motion Gun, also happen to be the ship mode's forward section. And indeed after the final battle of both the movie and the series, the Macross never transforms into ship mode ever again as it's stuck waist down in a lake.
One chapter of D.N.Angel had Dark in control of the body after Daisuke went missing (sort of) on White Day.
Haji of Blood+ suffers from this in a way. After being prematurely awakened in 1972 Vietnam, Saya went on a bloody rampage and severed Haji's right hand. Though it was reattached, it is now permanently stuck in chiropteran form, and he constantly keeps it bandaged up.
In Fullmetal Alchemist: Conqueror of Shambala, Envy transforms into a dragon before going through the Gate to our world. Since alchemy doesn't work in our world, he remains stuck in his dragon form.
A memorable example in Lucifer came when the Big Bad sent a shapeshifting demon to kill Elaine. The demon in question killed her father and took his form to get close to her. After Elaine escaped, the Big Bad decided to stop said demon from interfering by trapping it in that form permanently. It literally begged for death at the end, as the man's memories started to overwhelm it.
Context is necessary to really appreciate the "begged for death" bit: It is immortal to the extent it had already outlived the death of a previous universe. There's no indication ANYTHING can give it release.
Courier's not the only X-character to suffer this. Nearly any character with a civilian form and a combat form will find him/herself stuck in powered-up mode. Colossus has been trapped in metal form, Iceman in ice form, Lifeguard in half Shi'ar form, Shadowcat as intangible (not shapeshifter but worth mention), Wolfsbane as at least part wolf, and so on.
Parodied by the She-Hulk graphic novel. Jennifer is told by Reed Richards that he has bad news: She's stuck as She-Hulk forever and can never change back into regular human Jennifer. After a Beat Panel, She-Hulk inquired "So what's this bad news"? It turned out that Reed was lying anyway since he's a dick.
A Skrull named Khn'nr was to be used in the infiltration of Earth in Marvel's Secret Invasion event. He was mode locked into the form of Captain Marvel, a long dead hero, and brainwashed into thinking he was him from the past. When he found out, he rejected his part in the invasion, and now is trying to work against it.
Similarly, quite a few Skrulls who took the form of Dr. Hank Pym wound up losing themselves in the role/being overwhelmed by the personality of the original, and had to be put down.
During the period when the entire Skrull race lost their powers, they were stuck in whatever form they took at the time. Those whose jobs involved snaking through drainpipes, for example, had to be cut out. The villain who actually initiated the change was in a suit that he thought would protect him from the effect. It did not, leaving him permanently trapped inside, in agony because the "suit" was not hollow, but loaded with internal machinery that he had oozed around and between.
In another example, some Skrulls who invaded Earth were brainwashed into becoming cows, rendering them harmless. Unfortunately, the cows were then slaughtered, and those who ate the meat either died or gained powers along with a virus. That's the origin of the Skrull Kill Krew. Team member Riot was stuck in a monstrous bug form for an extended period of time.
Other instances include Manhunter being reduced to amorphous goo by a neurotoxin, being trapped in Aquaman's body and freaking out, and an Elseworlds story where all the heroes lost their powers, leaving him in his natural Martian form with all of the suck and none of the blessing.
The evil White Martians are initially dealt with this way by the JLA. They are given human form and have their memories removed, effectively trapping them as humans. Needless to say, problems occur when a few of them get their minds back and remember how to shapeshift.
In Transformers: Spotlight, Soundwave gets locked by the Decepticon Pretenders when he tries to stop them after learning the full extent of their plans.
Whether this is good or bad depends on the form he's stuck as. Though when he recently got stuck as Banner... he proved to be dangerous even without the Hulk.
It's all a matter of perspective. Hulk would love to be mode-locked and never turn into Banner again. Banner would love to be free of the Hulk, but at this point knows better. Both absolutely hate the idea of being mode-locked as the other.
A story arc in Legion of Super-Heroes involved Durlan shapeshifter Chameleon Boy getting stuck in his usual form and undertaking a dangerous journey on Durla to a place that would restore his shapechanging abilities. His father, who had long ago been Mode Locked into human form by a disease, came with him, but only to provide support; he had adjusted to having a permanent form and didn't want his abilities back.
Doctor Strange foe Dormammu's sister Umar used to be the Stronger Sibling. Then Umar gave birth to her daughter Clea while transformed as a human. To Umar's chagrin, this locked her into human form forever, which made her weaker than her brother.
In With Strings Attached, it happens to George thanks to a flaw in the ring that normally allows him to shape change at will. He gets stuck as a pigeon in New Zork and is not happy about it; he uses his ring more cautiously for a time after it happens.
In The Return of Hanuman, one of the conditions for Hanuman to be reincarnated as a human on Earth is that when he turns back into the original Hanuman, he could not turn back into human form.
In Howl's Moving Castle, Calcifer warns Howl that the more often Howl changes into his semi-avian form, the greater the chance that Howl will lose his humanity and never be able to change back. Other wizards in the film are mentioned to have done this deliberately in order to help with a war effort; Howl notes that by the time the war ends, they'll have forgotten that they were ever human to begin with.
Tobias gets stuck in red-tailed hawk form early in the series. He still manages to be useful to the team, though, and he eventually gets the ability to become human again, although the hawk becomes his default form from that point on: If he stays human for longer than two hours, mode lock applies again and he loses his ability to morph at all.
Besides the intervention of a Reality Warper (which is how Tobias got his ability to morph back), the only other known way to escape being a nothlit is if your animal form undergoes a natural metamorphosis. Once this happens (as when Cassie was stuck as a caterpillar but became a butterfly), your biological clock is reset and you have 2 more hours to change back.
Sixth Ranger Traitor David was deliberately lured into a pipe and then stuck there until he was trapped as rat. He is later dropped off on a deserted island with no-one but real rats for company.
The Andalite Aldrea forsook her people after the genocide of the Hork-Bajir, choosing to permanently become a Hork-Bajir and marry one of the survivors.
Two Andalite characters become nothlits in The Andalite Chronicles: Arbron, trapped in Taxxon form, and Elfangor, who deliberately becomes human and even has a human kid. But said Reality Warper later turns the latter back into an Andalite with full morphing abilities.
In a more horrifying example, if the Animorphs reach the 2 hour mark while demorphing, they can be stuck mid demorph, as horribly deformed human animal mashups. Fortunately, each time this has happened they managed to fully demorph.
This also happens to Aftran, Menderash and the ENTIRE TAXXON RACE
"Yennorks" are werewolves born with permanent mode-lock. Angua had a sister Elsa who was unable to turn into a wolf and her brother Andrei passes himself off as a sheepdog because of his inability to take human form. She makes it clear to Carrot that this doesn't make them a human and a wolf, they're both still werewolves, just werewolves unable to change.
Also used more conventionally within the series: Angua frequently worries about the psychological effects of becoming a wolf, fearing that if she stays in wolf form too long, she will forget how to be human.
Shown with her father, in particular, who is slowly forgetting how to be human. Mentioned also that the human/reasoning side becomes less powerful the longer they're in Werewolf form, while the senses fade in human form.
Borrowing can also cause this, in a way; if a witch borrows an animal's mind and stays there for too long, she'll forget she was ever human and it'll take a powerful witch to bring her back.
An unusual variation occurs in the Switchers series. The titular shapeshifters lose their powers at 15 (which is to say, Midnight on the morning of their fifteenth birthday) and are stuck as whatever they happen to be at the time. This issue is directly and pointedly addressed in the second book, appropriately entitled Midnight's Choice.
Several people in the Kiesha Ra series by Amelia Atwater-Rhodes have had their animal forms "bound" so that they can't transform, some intentionally as punishment. It's not a pleasant process.
Though none have appeared in published canon, Word Of God says that a shapeshifter who spends an extremely long period of time in his or her animal form can get modelocked as the animal. This is referred to as going feral.
Used in Tamora Pierce's Immortals quartet. A strange black hawk turns out to be a powerful mage after being given enough drugs to knock out a human - he was so sick in animal form that there was no way he could do anything, much less change back. Daine also gains the ability to shapeshift later on, but often can't shift back if she panics or forgets about her human self.
And she has the ability to morph into an immortal, but not the ability to change back.
This is a subplot of a Polish story "Academy of Pan Kleks".
Wilhelm Hauff wrote a story "Halif the Stork" where you need a formula to transform, but will forget it if you laugh.
In Harry Potter, Hermione puts Rita Skeeter, the nosy reporter who isn't above ruining people's lives by writing bald-faced lies about them, into a jar that makes her unable to transform out of her animagus form, a beetle. The jar is just sealed and enchanted to be unbreakable — thus, if she tries to change back... well, she'd be too big for the container.
Also, Dumbledore has mentioned in The Tales of Beedle the Bard that anyone other than an animagus that tries to polymorph themselves would permanently become an animal, unable to use magic to change back.
This also happens to Tonks although it's not too dramatic. Tonks is unable to use her metamorphmagus skills when she becomes depressed about her love for Remus Lupin and thus gets stuck looking rather like a girl version of him.
Dracula, unlike modern vampires, was not killed by sunlight, however he was unable to change shape from Sunrise to Noon, or from Noon to Sunset (he had one chance to change shape at noon exactly, and of course he could so at anytime at night).
In The Silmarillion This happens to Morgoth — "And then he took the form he had worn as the tyrant of Utumno — a dark lord, tall and terrible. And in that form he remained forever after."
Likewise his protégé, Sauron, after the destruction of his physical body in the drowning of Númenor. Even after he regained physical form, he could no longer take on a fair-seeming appearance.
This may be due to a recurring "you become what you do" theme that seems fairly consistent with regard to the Valar and Maiar. It may have something to do with why the Valar themselves increasingly become shut-ins on their own continent as well, though the text does not explicitly state so. If so, the entire existence of the Valar and Maiar within Arda can be seen as a progressively restrictive Mode Lock as time moves on. More specifically, evil affects the Ainur like a degenerative spiritual disease, progressively reducing their powers over time.
The Wizards in The Lord of the Rings, on the other hand, were good Maiar in a voluntary Mode Lock in the shape of old men — in fact, they were placed into real flesh-and-blood bodies, instead of the usual fana shapes of Maiar. This was so they would use knowledge to advise and encourage other peoples, rather than use their full abilities to grab power or cow people into submission. Didn't work with Saruman.
In the Earthsea books by Ursula K. Le Guin, wizards who spend too much time shapeshifted into animal forms can forget their humanity, especially when distracted by the animal's power of flight or ability to freely range the oceans.
The succubus lead character in Richelle Mead's Georgina Kincaid series suffers a mode lock, along with all of the other demonic immortals in Seattle, when their supervising Archdemon goes missing (summoned and bound by his lieutenant, with help). Georgina is lucky enough to be in her default form when the lock begins — another succubus is not so lucky and gets locked into a completely different body. This stasis removed definable abilities such as shape-shifting and aura perception due to their being normally 'distributed' via the Archdemon, but their connection to hell - and their immortality — remained.
In the Outernet books by Steves Barlowe and Skidmore, the shapeshifter-characters Sirius and Vega are trapped in the forms of a cat and dog, respectively. This remains throughout the whole of the series (with two brief exceptions).
Mikey McGill in The Skinjacker Trilogy gets stuck in his hideous monster form whenever his negative emotions overwhelm him, and it usually takes some sort of trigger to bring him back to normal.
Inverted in Dragons Winter when Karadur is locked in his human form (he's a dragon shapechanger). Later in the novel, Hawk is also so locked. Her alternate shape should be rather easy to guess.
In Prospero's Children by Jan Siegal, a sorceress with the ability to turn into a wolf would use her form to hunt humans for sport. One day she met a wizard who cursed her to remain in wolf form permanently until she could repent for her evil ways. After several years, she sought out the wizard so that she could show him that she had changed, but the wizard no longer had the power to change her back.
Curse Of The Wolfgirl reveals that werewolves are unable to shift out of their human forms if there is a lunar eclipse. Then things get worse when the Big Bad of the book finds a spell that can simulate an eclipse and conspires with a bunch of hunters.
In Mikhail Akhmanov's Arrivals From The Dark series, the Metamorph species is able to voluntarily shapeshift. Their normal form is that of an amorphous blob. There are a few individuals who have a mutation that locks the individual into the first transformation for life. At that point, only slight changes are possible. These usually become spies among other races, able to slightly alter their appearance within the confines of the race. The observer on Earth took on the appearance of a human male. He's able to change into other males of any human ethnicity but not females due to radical physiological changes.
The title villain of IT by Stephen King can be forced into one form if several people all think of it that way at once. Like a Giant Spider.
There's a German children's book (main character's named Agathe) which involves witches, shapeshifting into cats and a "stay-a-cat-powder".
Voluntary shapeshifting and other forms of Transhumanism played a big role in human society in John Ringo's Council Wars series until the beginning of the war imposed a Mode Lock on everyone. Results varied from unfortunate through unpleasant to instantly fatal depending upon how long the person had intended to stay transformed and how survivable the chosen form was when techology was lost. Both sides try to recruit people with useful shapes, despite the villains' purist ideology. Even though shapeshifting is a thing of the past how people deal (or fail to deal) with their mode locks remains a plot point for years afterwards.
Live Action TV
In Doctor Who, the Doctor's TARDIS possesses a "chameleon circuit" which, under ideal circumstances, disguises the machine by making it appear to be an inconspicuous object in the time and place it's parked. Due to a malfunction he's never been able to fix, however, it is permanently stuck in the shape of a 1960s British police box. He did fix it for one story, but then it broke again. It's later stated that he doesn't even try to fix it anymore, because he likes it that way. Other Time Lords that appear in the series have TARDISes with normally functioning chameleon circuits.
In the Doctor Who Magazine strip, shapeshifting companion Frobisher got stuck as a penguin for a while. Even before and after that, however, it was the form he most preferred.
Although, now it's stated that the chameleon circuit is working properly. It analyzes a huge area of its surrounding environment, extrapolates an inconspicuous form for it's exterior, then shifts that appearance to.....a 1960s British police box. Every time. But it is, technically, functional.
In an episode of Star Trek: Deep Space Nine Garak uses a device on Odo that prevents his shapeshifting. Since he cannot take his natural liquid form, his body begins to deteriorate. Later, he had his powers taken away by his people for a surprisingly long amount of time (unusual for this trope): about half a season.
Still later, in the latter days of the Dominion War arc, all of Odo's race (including Odo) fall under the influence of a disease which makes them unable to liquefy, slowly killing them as above. This is revealed to be a biological weapon invented by the shadowy, rule-evading part of Starfleet. Unfortunately for their plot, they had to infect the race through Odo, which means the good guys race to find the cure in the nick of time.
This happened to the character Tommy Oliver in Power Rangers Dino Thunder when the actor playing him had real life commitments; the character was stuck in his suit, then invisible for the best part of a season.
In one episode of Dengeki Sentai Changeman, episode dealt with Hiryū Tsurugi is stuck in his Change Dragon form. Unlike in other instances where it is treated as a minor annoyance at best, here it is treated as a real threat.
On Angel, during the Pylea arc, Angel in the other dimension would get stuck in the "super" version of his vampireness. The whole demon would emerge, instead of him just vamping out whenever he wanted. He would be stuck as the Van-tal for a really long time, until he could calm down.
It's heavily implied that the Van-tal demons were the original progenitors of the Vampire race. Which would mean that, by definition, vampires are themselves demons always mode-locked in a human body.
After Illyria, a primordial Old One had her powers mostly drained, she was effectively trapped in Fred's body. She did retain the ability to shapeshift... but only into Fred's human appearance.
In Buffy the Vampire Slayer, Amy the witch turned into a rat and couldn't turn back for 3 seasons. She was never really quite the same...
In the the Halloween Episode "Fear Itself", Oz is forced to live out his worst fear, which results in him suddenly starting to transform into a werewolf and getting stuck halfway.
LOST: Happened to The Man in Black twice. First, after killing Jacob he got Mode Locked in Mode Locke. He could still change back to the Smoke Monster form, but couldn't assume other human forms anymore. Then, the removal of the Island's cork in the Finale took that ability away as well, Locking him in a mortal body as opposed to the invulnerable Smoke form, allowing the Heroes to kill him.
On Fringe, the shapeshifter at the beginning of Season 2 is stuck in the body of Agent Charlie Francis after its shapeshifting device is broken. The shapeshifters' devices are unique so using another's would be pointless. The only way out is to complete its mission and go back before it is too late. This doesn't happen because Olivia Dunham shoots it in the head.
Happened to David Banner one time on The Incredible Hulk, but in an incredibly strange way: in the two-parter "Prometheus", Dr. Banner got a little too close to a radioactive meteorite, and got stuck halfway between himself and the Hulk. For the duration of the Shapeshifter Mode Lock, he retained his consciousness and intelligence, but had difficulty concentrating and focusing, and also possessed a fair portion of the Hulk's strength. Later in the same episode, he effectively got Mode Locked AS the Hulk after being captured and imprisoned by the military; his futile attempts to break free of his cell kept him pissed off enough to stay green.
Played for laughs in an episode of Mystery Science Theater 3000, where a shapeshifting witch visits the satellite and loses control of her powers. She eventually winds up stuck as a bottle of bleach, though she can still communicate ("This is so annoying!").
In Sanctuary, a pregnant HAP's body will go into lockdown to protect the fetus, because shapeshifting also changes the protean's insides.
Some variations of the selkie myth have it that they can only go upon land a certain number of times before suffering permanent Mode Lock (either as a seal or human depending on what they were at the time).
Another shapeshifter myth: the Margotines are fey white cats that can change into pretty human females — or can confer a human female the power to turn into a white cat. However, if the woman is wounded while in this form, she can no longer change back to her true shape — and the Margotine cannot turn into a human any longer either.
In Exalted, if a Lunar Exalted's anima banner gets too intense, the Lunar will be Mode Locked to a handful of his most iconic forms until the anima banner dies down.
Ars Magica. Using Muto Corporem (shapechanging) spells is potentially dangerous. If a mage spends a month in the shape of an animal, they will start to act and think like that animal. Eventually they could lose their human personalities altogether.
In the Amber Diceless Role Playing, it's possible for someone using Shapeshifting to lose the knowledge of how to shapeshift or even the ability itself, stranding them in their current form. It's also possible for them to lose their personality and take on the personality of the creature or person they're imitating, so they don't want to change back.
Forgotten Futures. A magician who transforms into an animal may come to believe that they are that animal and prolong the spell until it becomes permanent and erases their true personality.
In Eberron, when the elves, most of which where the slaves of the giants rebelled, the Gyrderi, who where the free elves decided to help their kin. Being druids they had an ability called wild shape, which lets them shapeshift into animals. The giants trapped the Gyrderi, and their descendants in their wild shaped forms
In Rage (the card game based off of Werewolf: the Apocalypse), Tamara Lovegrove of the Seventh Generation can lock Garou into their current form as her special ability.
In Werewolf The Apocalypse itself, werewolves and other shapeshifters can "lose the wolf," permanently losing their Rage and becoming trapped in their birth-form, if they suffer too severely from depression.
The Mistika Makuta from BIONICLE suffered from this, after being exposed to the Pit Mutagen.
Powermasters in Transformers Generation 1, in theory, required their Nebulan partner in order to transform from vehicle mode to robot mode (in reality, all that was really required was to simply press a button. The toy was capable of working without it, but the partner made it a tad easier — not to mention the toys looked better with the partner attached).
More severe were the Action Masters, a series of non-transforming Transformer toys, stuck in their robot modes. The fluff explained that they had taken a substance called Nucleon, that had rendered them more powerful, more 'alive', but robbed them of their ability to transform into vehicles. To compensate for this, the Action Masters were all packaged with transformable accessories, such as weapons or vehicles.
In Golden Sun, the party visits Garoh, a village whose inhabitants, due to proximity to the Air's Rock, turn into werewolves in full moon nights. The mayor, though, as Kraden explains to him, he has absorbed too much Psynergy to go back to his human form.
In Prototype, halfway through the game you are given a "cure" that mode locks your ability to shapeshift your body into weapons. Oddly enough though you can still shapeshift.
In Warcraft III certain units can shapeshift, but the action is considered to be a spell, and costs mana (and sometimes also has a cool down). So, if a Dark Ranger casts Silence on a bunch of Druids of the Claw, then, well, no Bear Form for you. Similar things can be done with Druids of the Talon and Spirit Walkers.
A TFT walkthrough states that in one mission, hitting Ilidan with a bunch of damaging spells will have a high chance of distrupting his AI, preventing him from transforming into his demonic form when heavily injured.
In World of Warcraft, the black dragon boss Nefarian has calls out to specific classes with specific effects on those classes. The class call to Druids ("Druids and their silly shapeshifting. Let's see it in action!") will force all Druids to shapeshift into cat form and block them in it for a while. Feral specialised DPS druids may not be affected if they're fighting in this form anyway, but druid bear tanks, healers and spellcasters will be annoyed. Of course, at the time Nefarian was at the top of the food chain DPS-specced feral druids, and for that matter any non-healer druids, were so exceedingly rare in end-game raiding that Nefarian was probably fine discounting their existence entirely.
Not quite Shapeshifting, but in Kingdom Hearts II, abusing Sora's drive forms can lead to him accidentally drive into the infamous AntiForm. Along with weakened attacks, reduced damage resistance, no healing and no allies, Sora can't exit this form until his drive gauge depletes, making it a temporary (but still often fatal) example of this trope. And woe betide you if you gave yourself abilities that increase drive form duration.
The amulet of unchanging will mode lock the player. This is usually a good thing, since most transformations are unwanted.
The ring of protection from shape changers will mode lock all enemy shapeshifters, which renders them effectively harmless.
An example of the lock out version of mode lock: In the current-gen console game of Spider-Man3, if the player removes the black suit, they have to wait for a small amount of time before they can use it again.
In The Legend Of Zelda Twilight Princess, Link gets stuck in wolf form whenever he goes into a area covered in twilight. The only way he can change back, is by returning the 'Tears of Light' to the spirit in that area. Later in the game however Zant curses Link, locking him in his Wolf form (despite there being no twilight covering the land) Once removed, said "curse" can be re-applied and removed at will, allowing Link to change whenever he needs to.
Majora's Mask begins with this trope. Link is changed into a deku scrub until he gets his Ocarina back and learns to play the song of healing. Of course later on you can change at will by applying variousmasks.
In Neverwinter Nights 2, a certain wonkiness in the nature of things makes it difficult -in some cases impossible- for the local druids to shapeshift. It's a story element and has no effect on the combat effectiveness of your party's druid.
Towards the end of Metroid Prime 3: Corruption, Samus goes to Phaaze, the source of all Phazon. However, the concentration of Phazon is so high that Samus gets thrown into a dangerous Hyper Mode has to drain all of her energy tanks in order to prevent instant corruption. And even that doesn't stop the problem since it just delays the inevitable where sooner or later, the Phazon will consume Samus so it becomes a Timed Mission from the landing to the end of the Final Boss battle.
Leader units in Transformers War For Cybertron get to inflict this on opponents as an ability, known as Disruption. In addition to causing damage, it forces anyone in robot mode into vechicle mode and vice versa, for a set period of time. This can be either pointless or deadly, given that the opponent has weapons in both forms but also can't access all of their abilties. A Scientist without his jet form will not last long.
Grimlock suffers from this in the sequal Transformers Fall Of Cybertron. Shockwave modelocked Grimlock into robot mode as a failsafe until he could fully control his test subject. Unstoppable Rage can let Grimlock temporaraly override the mode lock.
In Ys II, Dalles locks you in monster form halfway through the game, and you must go on a Fetch Quest to change back.
The player's mecha in Thexder 95 is unable to change into jet or tank form for the first few levels.
Slightly Damned: Kieri's "bunny curse" occasionally kicks in by itself and is difficult to revert.
College Roomies from Hell!!!: Roger's mom is stuck in furry form (and happy that way), and tells him that it will eventually happen to him too.
In El Goonish Shive, Grace and her brothers could each shapeshift partially or fully into whatever animal's DNA was thrown in with their human DNA. Since the brothers were a threat at the time, Grace hoped they wouldn't be able to turn back to normal if they were hit with Tedd's transformation gun. They dropped that plan when all it did to Grace was give her new forms she could shapeshift to.
Which brings us to Vlad, who was in a de facto mode lock as his attempt to turn human nearly killed him; he ended up stuck in his hybrid wolf/bat/falcon form. He's unlocked after Ellen transforms him into a woman, but the idea of losing human form again bothers her so much she refuses to be transformed into anything else. It's implied that she rejects everything about her old form, including her former gender.
Also the TF Gun has a built in mode lock related to it's original purpose: Transformations normally only last 30 days, but If you get pregnant while transformed, you stay female. (It's implied that you can always use the gun again after giving birth.)
In Jix, the main aliens (the Ambis) have the ability to going from cute and fuzzy to large and pointy beasts. This is known as their feral form. One of the villains, Maricax was introduced in his feral form, unable to revert back to normal. That is...until another villain named Kelelder gives him super healing ability.
Beast Wars and Beast Machines, not having the secrecy restriction of other Transformers series, invented excuses for the cast having to adopt and stay in alternate forms. In Beast Wars, it was that all of the robots would be debilitated by energon radiation and that organic-skinned beast modes would protect them (an excuse as good as obsolete by the second season, but by then everyone had vehicle modes that gave them the advantage of speed or flight anyway). (In at least one instance, having serious damage to the robot body prompted the internal system to force a Transformer back into Beast Mode.) Beast Machines had the Maximals stuck in beast mode until they learned how to use their new techno-organic bodies, and had to use their alternate forms to hide from the ubiquitous wardrones.
There was also an episode where Megatron has a beam-like device that trapped characters hit with it in Beast Mode. This lost the Maximals a chance to contact Cybertron.
The Transformers cartoon, specifically the episode "The Autobot Run" had the Transfixatron, a weapon that trapped most of the Autobots in their vehicle modes (all land-based, all unarmed), making them very vulnerable to the Decepticons. Later episodes would imply that Mode Lock could also be the result of heavy damage, and the Headmasters were automatically stuck in vehicle mode anytime their partners (who transformed into their heads) weren't around.
Also in "Desertion of the Dinobots", a little bit of Phlebotinum called "Cybertonium", which all Cybertron-created Transformers (i.e.: not the Dinobots) need in order to function correctly, starts decaying, resulting in semi-hilarious malfunctions (Megatron in at one point waving his arms around wildly when trying to fly, Ironhide icing himself up). Jazz is shown as unable to transform from car mode (until Ironhide kicks him), and is later shown stuck part-transformed (robot upper body, still in car mode in lower body). During the episode, Perceptor actually says "Fortunately I am still stuck in microscope mode").
The Beast Wars example is a double-edged sword: by remaining in beast mode for too long, the beast instincts start to overwhelm the robot intelligence until they become feral. Certain Maximals who frequently remain in beast mode, notably Tigatron, are able to overcome this.
After even the normal Maximals were forced into permanent Beast Mode for an episode, they too learned the trick, and proceeded to teach the Predacons a lesson in Beast Wars.
Yet another Beast Wars example: Optimus Primal became mode-locked in "Gorilla Warfare" thanks to Scorponok frakking around with his cyber-bee. Naturally this backfired, turning the normally peace loving Primal into a Berserker.
He then created and subsequently murdered a clone to get THAT T-Cog and installed it in himself. Bumblebee managed to retrieve his original T-Cog eventually, though it was heavily, but not irreparably, damaged in the process.
Ben in Ben 10 has this happen to him a lot, since he's basically got a Black Boxfrom outer spacepermanently attached to his wrist. Even leaving aside that when he uses it, it turns him into the alien he chooses (or the one it chooses instead) until the timer runs down, then turns him back human until it powers up again, there have been a number of Mode Lock incidents, generally involving a weaker alien such as Grey Matter or Ditto.
As a variant, Kevin winds up submitting to Mode Lock after overuse of previously-acquired Omnitrix-fueled transformation — what Vilgax lovingly calls a "misshapen amalgamation". By the time Alien Force rolls around, he returned to human form in time for his escape from the Null-Void.
Seems to have happened to Kevin yet again in Alien Force, since now he's part concrete, metal, diamond, and other stuff.
Also happens to Albedo, Ben's doppelganger in Alien Force, who turns into Ben to steal the Omnitrix from him, and uses a crude Omnitrix copy to access the alien forms. When they fight, the magnetic interference caused by the meeting of two Omnitrixes causes Albedo's colors to change. When Azmuth appears to tidy up the whole mess, he takes out a part of Albedo's fake Omnitrix, cursing him to stay in human form (which he absolutely hates).
Also a plot point in the episode "Con of Rath", wherein Ben spends basically the entire episode in the form of the titular alien (which debuted in said episode). This was due to interference inadvertently caused by the Luodan Prince Tiffin, whom the crew have to escort in a supposed peacemaking effort.
In Visionaries, the title characters are futuristic knights who can transform into animals. During the episode "Lion Hunt", one of the Visionaries gets stuck in animal mode.
An episode of The Mask: The Animated Series featured a gypsy fortune-teller that tricked Stanley into giving her the Mask which she then used to power another magic Mask that gives enormous powers. She then discards the Mask thinking it's now useless and Stanley puts it on... only The Mask is stuck in the form of a sterotypical Scot (complete with kilt). He then gradually obtains the rest of his forms (starting with the most useless ones, as The Mask himself lampshades).
This is the entire purpose of the Plasmius Maximus in Danny Phantom. It has been used on both Danny and Vlad, modelocking them into their human forms. Other weapons/containment devices have this as a side effect of sorts.
Don't forget the episode "Forever Phantom", in which one of Jack's weapons zaps Danny and Amorpho, forcing the former to stay in his ghost form and the latter to stay in Danny's human form...even though Amorpho still manages to (with difficulty) shift a bit more before being cured. It's a Long Story.
Between episodes 21 and 23 of Wakfu, Adamaï, a young shapeshifting dragon, is stuck in Tofu form after being swallowed whole and then regurgitated by Igôle (a powerful beast reinforced by Xelor magic, which is what interfere with Adamaï's power). This is a case where the modelocked form is small and weak, Tofus being Ridiculously Cute Critter birds.
Gargoyles - Puck is mode-locked into one shape, that of Owen Burnett, by Oberon after rebelling against his king. Given that shapeshifting is a major part of Puck's tricks... this sucks. The punishment also included eternal banishment from their homeland Avalon. Puck wanted to spend more time living as a human in the human world, and Oberon gave it to him.
The only way he can get mode-unlocked is to train or protect Xanatos's son, Alexander.