This Skittles commercial. Three teens sit on a rainbow and enjoy their Skittles while looking down below: Teen #1: "Hey, what if this rainbow doesn't exist and it's just in our imagina-" the rainbow opens under him and falls out of the sky in mid-sentence while the other two teens look on, followed by the slogan "BELIEVE THE RAINBOW! TASTE THE RAINBOW!"
Anime and Manga
This is the premise of Noein. Two competing potential futures are battling to influence the past and cause their rival to cease existing. The big bad is eventually killed by everyone refusing to acknowledge his existence.
In Hellsing, this is how the mighty Alucard met his demise. He absorbed the soul of Schrodinger - a Nazi catboy, whose prime trait was an ability to exist as long as he could recognize himself. Once absorbed and integrated into Alucard's pool of souls, Schrodinger could recognize himself no longer and disappeared along with Alucard; for a little while.
A duplicator story in Calvin and Hobbes ends this way. Calvin made a clone of his "good side" to do his chores and go to school for him. They got into an argument, and the clone disappeared as soon as it had an evil thought.
In Bloom County, Oliver Wendell Jones creates a math formula which explains the existence of everything everywhere...except penguins. Opus disappears, and then reappears when Oliver notices a mistake in the formula and rewrites it.
Rebis: I've come to ask the question. One of you must have the answer. Why is there something instead of nothing?
Priest in Black: I am a liar and I do not know why there is something instead of nothing.
Priest in White: I am an honest man and I do not know why there is something instead of nothing.
Rebis: Tell me then, the Priest in Black, why is there something instead of nothing?
Priest in Black: There is something instead of nothing.
Rebis: Then you can't possibly exist.
One Nodwick story sees the party 'defeat' a lich by smacking it in the head with a magic club that makes you smarter (called, the "Clue-By-Four"). The lich, realizing that its status as The Undead only prolongs its own agony and causes it to lash out on others thus making the universe a crappier place to live, promptly reasons itself out of existence.
Mentioned in Unfamiliar when Alex tries to dispel Louise's notion that she is incapable of magic.
At the end of The Flight of Dragons, the main character defeats the Big Bad wizard by telling him magic is impossible. Without belief, magic doesn't work and knowledge of science is incompatible with belief in magic.
In the Fight Club movie, Tyler disappears when the Narrator "kills him". That was a key point in the movie that the Narrator managed to take control of his own mind.
The Trope Namer is Hitchhiker's Guide to the Galaxy, where God's existence is disproved because the Babel Fish is just too handy to have evolved naturally. As the argument goes, God refuses to prove his own existence, because "proof denies faith, and without faith I am nothing." Man points out the Babel Fish as a "dead giveaway", causing God to vanish "in a puff of logic." The Guide then says that this argument has been universally rejected on the basis that it is profoundly stupid; just to start, the Babel Fish being made unnaturally doesn't mean God had to have made it. Of course, that doesn't stop in-universe writers such as Oolon Coluphid from cashing in on the argument.
In his 'Meditations on First Philosophy', Descartes actually this by noting that if he wasn't thinking, then perhaps he would not exist. He goes on to state that, regardless of what else he may be wrong about, he must at least exist in the capacity of an entity that is capable of being wrong; if he doesn't exist in at least that capacity, then it's not his problem.
Belafon sat down with his feet dangling over the edge of the rock. "Look, don't worry," he said. "If you keep thinking the rock shouldn't be flying it might hear you and become persuaded and you will turn out to be right, okay? It's obvious you aren't up to date with modern thinking".
The auditors believe that because mortals have a finite life and that anything finite is insignificant compared to the infinite length of the universe, to develop a personality is to become mortal is to die. Any time an auditor starts using the first person, you can bank on it evaporating in a blue flame very shortly. The narrative notes that this isn't logical, but the auditors self-destruct too quickly to catch on.
Then there's that genie in Sourcery. The characters wish to go someplace. Cue them all sitting inside the genie's bottle for the ride. . . .while they themselves are carrying it. The entire setup was banking on the universe not noticing what they were doing, which backfired once one of the characters pointed out the conundrum. Puff of Logic ensues.
In The Pedant and the Shuffly by John Bellairs, persuading hapless passersby that, logically speaking, they don't exist, is the wicked magician Snodrog's favorite trick.
In Fight Club, Tyler Durden disappears when Marla shows up to confront the narrator, the explanation being that Durden was his hallucination, and not hers.
In the Nightside novels, Tommy Oblivion, the Existential Detective, can cause things like this to happen.
On one occasion, he immobilizes a couple of ambulatory statues by explaining all the reasons why it should be impossible for them to move.
Hell to Pay has a literal example. Taylor points out to newly created God of Creationism, that creationism doesn't believe in evolution and that Creationism has evolved into Intelligent Design, therefore disproving itself.
"Oh bugger," said Chuck, as he disappeared in a puff of logic.
In the Fengshen Yanyi, a certain Bi Gan was coaxed by Daji to have his heart removed. He managed to survive without his heart until he was said that no one can live without a heart. He then collapsed and died.
The Dresden Files: Ghost Story has a non-fatal example. Harry, who is a shade, is riding in a car. It occurs to him that, as a ghost, he is capable of moving through solid objects such as walls. This causes him to wonder what's keeping him in the car — and as soon as he starts thinking this, he immediately slides through and out of the car. Ghosts in the Dresdenverse get most of their abilities through perception; since Harry had seen other spirits move through walls it became something he associated with them being able to do. Cars, on the other hand, he thought of as solid, because he rode in them often when he was alive. As soon as he started doubting whether or not it was, it ceased to be to his spirit form.
In the opening chapter of Bill Bryson's history of science A Short History Of Nearly Everything the author invites the reader to consider the molecular reality of their existence:
"It is a slightly arresting notion that if you were to pick yourself apart with tweezers, one atom at a time, you would produce a mound of fine atomic dust, none of which had ever been alive but all of which had once been you."
An apartment building built using the power of hypnosis. It would only remain standing as long as the tenants believed it was the best housing available. When interviewed, one of the tenants mentions that they used to live in a villa overlooking Nice. When the interviewer comments that the villa sounds much better, the tenant realizes it's true... and the building starts to shake. Fortunately, the man says "NO! NO! OF COURSE NOT!" Then the building changes back to normal, although some of the furniture (and the tenant) were a little shaken.
Tenant: Whew, that was close.
In a Sherlock Holmes sketch, Holmes stands up to being shot with his own gun several times, then explains that he'd had Watson load the gun with blanks. However, Watson then admits he forgot to change the bullets, and Holmes notices the huge amount of blood on his chest. He dies soon afterward.
Another Pythonic version of this trope was a short animation by Terry Gilliam of Rodin's The Thinker coming up with Descartes' "I think therefore I am." in a thought balloon. When a hand comes from off screen and deflates the balloon with a pin, The Thinker looks up briefly in horror, then disappears.
The final episode of Blackadder the Third has Prince George shot, then rising alive and well due to a Pocket Protector. When he attempts to show it off, however, he can't find it, remarks "Damn, must have left it on the dresser this morning", and promptly dies.
Done in the second season of True Blood in the case of the Nigh Invulnerable ancient Maenad, Maryann Forrester. It's revealed by vampire queen Sophie-Anne that Maryann's powers are directly tied to her beliefs, stating everything in the universe imagined itself into existence. Thus by believing in her god Dionysus and becoming a maenad she was immortal. In the end Sam tricks Maryann into believing her god has returned from her sacrificial ritual by shapeshifting into a bull and then impaling her in the chest. She is surprised when this happens, but rationalizes that if she is the final sacrifice for her god to come into the world, then she will accept it. She's shocked again when Sam morphs into his human form, questioning if there was even a god. This moment of vulnerability allows Sam to rip her heart out of her chest and crush it into the ground, finally ending her reign of hedonistic Mind Control.
Directed by Steven Spielberg. A crippled WW2 bomber returning to base no longer has landing gear, and the turret gunner is trapped under the belly, facing an agonizing gory death when they land without wheels. He desperately draws cartoon tires (a la Who Framed Roger Rabbit) but is running out of time. The tires serve to safely land the plane, but disappear when he faints in relief after he's safely out of the ball turret.
The Mark Steel Lectures profile of Rene Descartes (see above) includes a spot where Mark invites the audience to stop and think about whether they exist for twenty seconds (with "Life is But a Dream" playing behind) ended with a buzzer and the message "Stop Thinking Now!".
An episode of The Walking Dead has Rick have periodic telephone calls in an abandoned prison. A variation occurs on the final phone call, because once he discovers who his mystery callers are, the phone calls stop. How is this a puff of logic? Every caller was someone who was already dead, including his wife.
There is a spell called Solipsism, which can create illusory objects that behave exactly like real ones — but only if you believe they are real. If you don't believe in it, you'll fall right through that bridge the cleric conjured. This is loosely based around the (Real Life) philosophy of solipsism, which posits that the only thing one can be sure exists is himself, and therefore one's perceptions define one's reality.
It also plays with this trope with the spell Shadow Conjuration. It summons quasi-illusory monsters that are as powerful as the real thing if you believe they're real, and one-fifth damage if you don't. Like all things D&D, it can be whored to ludicrous lengths. One prestige class specialises in shadow illusions. Layer enough "reality"-boosting effects, and you can create illusions more real than reality. Cue humorous conversations:
"I cast a shadow fireball."
"I make a will save to disbelieve."
"Good. Take 150% damage."
The RPG TORG had the fantasy world of Aysle, where the local laws of reality stated that Your Mind Makes It Real to the extent that real people could cross a sufficiently convincing illusionary bridge.
The 3E Ravenloft description of Lamordia, land of mad scientists and skeptics, describes an optional phenomenon called the "Smothering of Reason", where Lamordians' prevalent disbelief in the supernatural occasionally causes spells to fail, magical creatures to weaken, etc.
The Fantasy Trip has a powerful Illusion spell. Illusions deal damage as though they were real, but can be dispelled by making a roll to "disbelieve", causing the illusion to vanish in a puff of logic.
The Illusion advantage in GURPS has an upgrade that allows it to cause small amounts of damage if the victim is convinced it's real. The GURPS spell Solipsism causes you to assume you're the only thing that's real and the guy swinging a sword at your head won't really hurt you.
The Knack Solipstic Defense allows you to ignore an attack entirely by believing it isn't real - since you have Legend, an element of the divine, what you believe goes. The other requirement is being unaware of the attack.
Aten, sole avatar of Akhetaten, The Titan of Light (Having devoured all the other avatars, who were not worthy of existing compared to his glorious self) is a version of this. He is so certain of the fact that his existence is the only one, the he is immune to any source of damage from anything. Your only hope of defeating him is to first debate him in philosophy and theology.
You can perform amazing feats as long as you keep them believable, such as conjuring money from your wallet, or causing lightning to strike during a thunderstorm. If you strain disbelief too much, or simply don't care and go all-out, it might still work, but you have a pretty good chance of getting smacked around by the universe at large on the grounds that Magic Does Not Work That Way. This is true even if there's nobody watching.
The setting explains that the main antagonists, basically The Men in Black, came into power through a centuries-spanning Evil Plan of convincing the world at large that no, brooms can't fly, and that yes, you can heal people by injecting them with complicated chemicals. They have caused mythical creatures like unicorns to go extinct by making people stop believing them, and caused places like El Dorado to cease to exist by creating maps to prove they weren't there. Conversely, there are some areas in the world where people still cling to their old beliefs, and thus the effect is reversed. In certain secluded spots in the Middle Eastern deserts, flying carpets and genies are considered normal, whereas airplanes are considered "vulgar magic" and a violation of common sense, and stop working as a result. By now it is clear that there is absolutely no difference between Science and Magic. Science is just Magic that most people believe in, and therefore everyone can use. The various 'Mystic' factions are trying to reverse this trend by getting people to believe in their path instead of Science.
Mage: The Awakening, Mage's reboot, while doing away with the Clap Your Hands If You Believe angle, also has an example of a Puff Of Logic; a certain Abyssal manifestation that amounts to a inescapable maze can be negated just by navigating it in a such a way to prove that it must have an exit and logical connections.
Toon. Characters are able to pull cartoon stunts such as walking on air, sawing through a branch and have it remain hanging while the tree falls away, painting a door onto a wall and opening etc as long as it's funny and they fail the Smarts check. It is a rule of the game that these feats will work so long as the characters don't realize that they should be impossible. That's right, the dumber a character is, the more easily they can get away with violating the laws of physics and common sense. The space opera-themed chapter of the Tooniversal Tour Guide supplement mentions a planet that is so dependent on this principle that it's off limits to visitors for fear that they might bring its whole civilization crashing down by pointing out that it can't work.
In Genius The Transgression, the inventions of Geniuses are literally powered by their creator's madness. If these creations are scrutinized by Muggles, these devices will stop working or malfunction.
In Macho Women With Guns, the Batwinged Bimbos From Hell can learn the "Distort Reality" skill; when they are attacked, they can focus their attention on something else (their nails, a run in their stockings, a cloud, etc) and since they've forgotten the attack, it can't hit. It will instead target one of her allies, who might not appreciate the gesture.
In Warhammer 40,000, the Chaos God Tzeentch is the god of scheming and ambition. He is THE Chessmaster in a universe of chessmasters, with hundreds, if not thousands of plans running at once, yet not one of his plans have ever succeeded. The reason is that he ruins his own, since it is very possible that if that he wins, or even has a single, solitary plan work, he may cease to exist.
In Tales From The Floating Vagabond one of the Shticks, the Newton Effect, allows a player character to point out something they believe should be impossible (such as magic powers or mad science) and have it stop working.
A variant happens in Gilbert and Sullivan's Ruddigore. The Baronetcy of Ruddigore is cursed - its occupant must commit a crime every day, or die, dragged off by the ghosts of those previously killed by the curse. Then the current Baronet realises that suicide itself is a crime, and since not committing a crime is tantamount to suicide, this too counts as a crime. Therefore, none of the people killed by the curse should have died; as soon as this is realised, the former Baronets simply pop back to life, just in time to Pair the Spares with the female chorus.
During the Bohrok arc of BIONICLE, Pohatu and Onua, both the strongest of the Toa, struggle to break a rock wall to escape before the room fills with lava. However, they're unable to make a single chip in it. Lewa realizes that there's nothing the two of them are unable to smash, which means the wall cannot exist. As soon as they stop believing the wall exists, it disappears.
Sure enough, the projectors of said wall, the Bahrag queens, are able to create illusions at will. Several of the Toa again end up fighting false opponents, being helped by their teammates that can see which Bahrag aren't real.
Divine Divinity had two skeletonMooks discussing the scientific impossibility of their existence. One of them remarks that realizing the truth would be quite unpleasant and the other agrees to drop the issue. Still, the seeds have been planted and they both collapse immediately after.
Planescape: Torment. Being set in Planescape (mentioned above), everything works this way. Several notable examples include:
The Nameless One recovering a memory in which an earlier incarnation manages to prove to a Sign of One devotee (a sect of solipsists) that he does not actually exist... Said man then disappears, much to the delight of the crowd who has gathered to watch the debate.
If the Nameless One gives his name as 'Adahn' frequently enough, a man named Adahn appears in-game. He only exists because the Nameless One's been claiming to be him, after a fashion, and if you point out as much to him... poof.
In Flight of the Amazon Queen, you encounter a gorilla blocking your way. How do you get rid of it? By telling it that gorillas only live in Africa.
In the first third-season episode of Sam & Max, if Sam looks at a readout on one of Mama Bosco's computers, he will say that according this they should not exist, at which point both characters disappear for a few seconds before coming back. Given that they are a talking dog and a rabbit-like thing, the readout is partially true. On the other hand, their entire world runs on the Rule of Funny.
In Silent Hill 1, Lisa disappears in a puff of blood when she realizes that she is the same as the other monsters around her.
Super Scribblenauts allows the player to summon various deities. It also allows you to summon an atheist who upon seeing a god will immediately start displaying the "thinking" icon and run towards it, causing the god to die on contact.
An in-game book in The Elder Scrolls games has a story of a young suitor who is vying with other men for the hand of a lady. By consuming a powerful Fortify Intellect potion, he creates a mathematical proof that a rival suitor does not exist. Said rival promptly disappears...
Dwarf Fortress occasionally does this to itself: Parameters to define how something exists in the world map are quite different from those used in the parts where the game is actually being played, and as soon as something from the former enters the latter, it gets subjected to reality. If this happens, best case scenario is a centuries old king dies from old age as soon as they step into your fortress. Worst case? You get a frame to watch the surrealistic scenery you embarked into before physics apply and your dwarves get swallowed by the collapsing earth.
Vanishing in a puff of logic is a consuming peril for the witches in Umineko no Naku Koro ni, who are largely only as real as they can convince other people they are. Witches are therefore continually performing horrendous deeds in order to be recognised, while trying to undercut the reality of their rivals. At one point a witch uses magic to disprove her own existence, which has a result like reality Dividing By Zero.
Red Mage also tried this on a giant muchearlier on, and failed spectacularly.
He uses similar reasoning in a later strip to deny the existence of 'Megahedron', his supposed avatar of the gods, claiming that it is really just a figment of his own imagination. His reasoning was that, if Megahedron was a figment of Red Mage's own mind, then he'd be smart enough to be able to convince him of his existence, a task at which he had so far failed (Megahedron's own suggestion). When Red Mage conceded that point, he went on to say that since Megahedron had convinced him with that argument, then that means that Megahedron is a figment. The only problem with that (which RM is too busy ignoring Megahedron to listen to) is that by that logic, Megahedron did not convince him. So that should convince him, which should not convince him, which should and oh dear, I've gone all cross-eyed.
Captain SNES: The Game Masta attempted this three times... twice in canon and once as a side-story. The first In-Canon attempt failed miserably, while the side-story one succeeded. The second time in canon, the victims did it to themselves accidentally.
Stickman and Cube did this in the early "Cube Disappears" arc, which ended up creating the comic's first recurring villain.
In PVP, Scratch Fury succeeds exactly where Red Mage failed in this strip. He does it again, to a chimera this time, in this strip. This time, however, it doesn't go off until he corrects an error in his equation.
It occurs in this Sverd Shelgr strip, though it's more of story mechanics than true logic.
In Dr McNinja, the good Doctor is knocked out at his parents' house, and his family is kidnapped. When he awakens, Doc discovers that the (cooked) turkey they were going to have for dinner is now walking around and talking. He correctly surmises that this is a hallucination brought on by his injuries and guilt over endangering his family. After he leaves to go save them, the turkey continues to move and speak for a few seconds, before realizing that it can't do so when no one is around to hallucinate it. It promptly falls over, inert.
Happens here in Scenes From A Multiverse. A mystic goat said 'there is a high probably that I myself do not exist'. When a reporter asks him what would happen if that were proven true, he disappears.
One strip in Melonpool had Sam the dog writing a letter when Roberta comes by and talks with him, asking how a dog can write a letter with no thumbs. Almost instantly his handwriting becomes scribbles and he looks at her in annoyance.
Please refrain from reminding the agents that this is chemically impossible. They already know that, and they don't care. However, if you remind them of this, it may stop working. Then they would have to kill you.
In Edds World, the three main characters get a car and get it to fly by "using the Z-Gear of course". It goes up to very trippy music in which Edd and Tom start dancing in the car. Matt then shouts "how can a car fly?" It then falls down to land on top of a plane. This trope is then inverted when the pilot wonders whether his '''plane''' can fly. Then this continues and when the car gets there, several planes are now below them (each having doubted their flying capability).
In the College Humor spoof "ProfessorWikipedia", a student asks Professor Wikipedia to tell everyone his knowledge about her. Another student points out that she's "not notable" even though she edits the yearbook, and she vanishes into nothing while screaming.
The Horror behind SCP-946 is that it can do this - potentially on a massive scale, like changing the laws of physics.
This becomes a plot point in Tiny Toons, where Elmer Fudd explains that as long as they don't realize there's no ground beneath them, they will not fall. At the end of the episode, it help them cross a Broken Bridge.
SpongeBob SquarePants: Spongebob and Patrick as keeping warm with a fire, Patrick questions how they have a fire when they're underwater, and it immediately goes out and then lights up again later when Patrick kicks the tinder.
An episode of Histeria!! had the kids annoying René Descartes ("I Think, therefore I Am") so much that he couldn't think. As soon as he realizes this, poof!note For those not in the know, his famous statement refers to the fact that he is capable of thought, in general, and thus he — or at least his mind — must exist in some form because the thoughts must be originating from something. But of course that's not as funny as a person spontaneously poofing out of existence.
In The Simpsons, Grandpa sees Tennessee Ernie Ford, an old celebrity whom he believes to be dead:
Tennessee: No, you just think I'm dead.
Abe: No, you're dead. I was your biggest fan. Look, I clipped your obituary.
"Phineas and Ferb's Quantum Boogaloo" has the Bad Future version of Candace disappearing in one of these after Phineas points out she'd be gone after the two Candaces undid that future. Also earlier in the episode (...or, alternately, twenty years later), Candace climbed a holographic tree and stayed in the air until she remembered it was holographic, at which point it turns static and she suddenly fell.
Done again in "Sleepwalk Surprise" when Doofenshmirtz see his dream demon while awake. When asked about it, the demon disappears in a Puff of Logic.
The Powerpuff Girls : In a musical episode, a gnome takes their powers to take the evil out of the city. Later, they regained them after learning that the gnome was a communist by feeding off the wants of the people to thank him from that point on and forcing them to worship him. They fought him later, winning by singing about how everything needs its opposites and chasing him off the edge of a giant vine. He then went into a non-singing speech saying "As I descend to the Earth, and I view the universe above me, I realize that life revolves, evolves... and dissolves, completely around the opposites. Therefore, I conclude... that I cannot... exist, in my Utopian... mind." When he hit the ground, all the lines became white and the colors became black, and he exploded.
An episode of El Tigre had Frida defeat a skeleton by pointing out there was noting holding it up, causing it to collapse.
During the intro to an Arthur episode, Arthur is talking about the upcoming story while Francine is flying around in the background. At the end of the intro Arthur turns around and yells to Francine that flying around like that is impossible. She immediately crashes to the ground.
Bender attempts to invoke this in Futurama with the Robot Santa. "Come on, everybody! He can't hurt us if we don't believe in him!" It doesn't work.
Also, when the crew is incapacitated by a cloud of toxic chlorine gas:
In the Super Secret Secret Squirrel episode "Quark", a sub-microscopic supervillain named Quark plans to flatten North America by "pulling out the bottom atom" of everything to make it all collapse, to make room for his own amphitheater (Canada) and a parking lot for it (the US). Secret Squirrel shrinks to subatomic size and lures him onto a dictionary, then points out the dictionary defines a quark as "a hypothetical subatomic particle". Secret manages to make the villain doubt his own existence enough that he vanishes into thin air. Science Marches On but it was relevant at the time that the episode was aired.
Invoked in a Foghorn Leghorn cartoon where he was playing hide and seek with the nerdy little male chick. Foghorn hid in a shed, the chick scrawled some equations on a piece of paper and then found Foghorn in a different location. Foghorn goes crazy, pointing out that he was hiding in the shed, at which point the chick shows him the equations and Foghorn is forced to admit that "figures don't lie". Foghorn and the chick walk over to the shed, and Foghorn stops as he is reaching to open the door, saying "I better not open it; I just might be in there".
In Batman: The Brave and the Bold, Batmite succeeds in getting the show cancelled for a darker one, but Ambush Bug points out that the mite is too silly a character for such a dark show, resulting in Mite being erased from existence.
South Park has an episode (involving a Tooth Mafia) where Kyle began questioning the existence of everything after having learned that the Tooth Fairy is not real. Near the climax, Kyle began questioning if he would cease to exist if he didn't keep thinking about his existence, and then promptly vanishes into thin air because he stopped thinking about his existence. He got better.
In an old Tweety Bird Cartoon, a cat (not Slyvester) flies after Tweety who comments that he didn't know he could fly. The cat only laughs at him for a moment before realizing that Cats CAN'T fly and plummets down to the ground.
Two men are standing on a skyscraper. Suddenly, the first one jumps down. The other one expects him to die horribly, but to his surprise, the first guy just bounces off the pavement, bounces a few more times, until he comes to rest. The second guy inquires how this is possible. The first guy explains: "It's all in your mind. You just have to think you're a rubber ball, and you will bounce just like a ball." "And everyone can do that?" "Yes, everyone, it's the easiest thing. Try it." The second man is curious and really makes the jump. All the time while he's falling, he thinks "I'm a rubber ball, I'm a rubber ball". Then, shortly before hitting the pavement: "But what if I was a tomato?" SMASH!
Inversion Punchline: "You're such an asshole, Superman."
Descartes walks into a bar, the bartender asks him what he wants, and he orders a beer. He drinks it, and the bartender asks him if he wants another beer. Descartes says "I think not", and disappears in a puff of logic.
Schroedinbugs are bugs in which a program works fine until someone looking at the source code realizes it shouldn't work, at which point it stops working. This occasionally happens. Mostly it's because it only works because of some strange bit of junk data lodged in the memory it references. When the program is edited, it references something else, which means that the little bit of randomness that made it work isn't there anymore.
Even more aggravating are the reverse phenomena: heisenbugs. The kind that are immediately apparent when you are running your code normally, but magically disappear when you try to fix or isolate them, leaving you with that nagging feeling that they can come back at any time, no matter how much work you've done to fix them.
It's common to wake up from a dream when you realize that you're dreaming. If you stay in the dream however, you can take over the place.
Similarly, if you become good enough at a skill that you're able to excel at it without even trying, if you actually stop to ponder how you're doing it you'll probably panic and start screwing up. (Playing the piano, for instance.)