Heavy Metal L Gaim: All what Daba wanted doing at the first chapter was delivering a cash card to a stranger. But somehow he got involved in a rebellion against The Empire and a war that spanned five planets.
Mazinger Z: In episode 46 Dr. Hell's had a pretty simple plan: send two Mecha-Mooks to infiltrate into the Institute and planting several nukes in Mazinger-Z's launching dock. Everything explodes, he wins. However that simple plan met complications due to his Robeasts and his troops running into abandoned babies, suicidal mothers, and at a point they got involved in a manhunt through a woodland.
Astérix And The Golden Sickle: Asterix and Obelix go to Lutetia to buy a new golden sickle for Getafix. They soon become involved in a mystery involving a gang of golden sickle traffickers, and end up being arrested by the local Romans several times.
Asterix: To think we only came for a golden sickle!
Shaun of the Dead. "Take car, go to mum's, kill Phil (sorry!) Grab Liz, go to the Winchester, have a nice cold pint, and wait for this all to blow over." Goes about as well as you'd expect in a zombie movie.
The poster for the 70's classic Dog Day Afternoon even describes the trope as its plot: "The robbery should have taken ten minutes. 4 hours later the bank was like a circus sideshow. 8 hours later, it was the hottest thing on live TV. 12 hours later, it was history."
Neatly done in Quick Change; the complex, intricate bank robbery that the protagonist and his friends have planned goes off without a hitch. However, what should be the incredibly simple matter of driving to the airport to make the getaway turns into a convoluted, mishap-ridden nightmare.
Adventures in Babysitting: Chris is babysitting when she gets a call from a friend who is stranded at a city bus station and in need of a ride home. Chris takes the kids (and their tagalong neighbor) along rather than leave them unattended while she's supposed to be babysitting them. Matters quickly fall apart from there.
The film Inception plays a tune from its official soundtrack as they plan their mission into Fischer's subconscious: "A Simple Plan." When they enter the dream and find that Fischer's subconscious has been weaponized and attacks the team, the entire mission has to be improvised. Not to mention that Cobb's dead wife is still haunting his subconscious...
Plan B: The plan is for Bruno to win back his ex-girlfriend by seducing her new, bisexual boyfriend, causing them to break up. Things start getting complicated when the Faux Yaystops being so fake...
Inverted in The Adventures of Huckleberry Finn. Breaking Jim back out of slavery would be an almost effortless operation for Huck and Tom Sawyer, but Tom insists on doing outlandish things like sneaking tools in through Jim's lunch, trying to dig an entrance hole into the (unguarded and unlocked) slave quarters with pocket knives, and trying to scare the Phelpses into staying inside so they can kidnap Jim without fear of getting caught. All on top of the long process of forcing Jim to keep a prison diary.
This is even more cruel (and hilarious) than it seems, since Jim has been legally freed by his owners days earlier, and could just walk out at any time. Tom doesn't tell him until later because he wants an adventure.
Donald E. Westlake likes to use this trope. The Hot Rock and, of course, What's the Worst That Could Happen? are two examples. The Worst is interestingly the one Dortmunder book in which the plan went perfectly.
Getting the Hrum gold in the Farsala Trilogy. It starts as a plan so simple it could probably be described in less than a sentence and evolves into about a third of Forging The Sword.
Exceptionally common in the Star Wars Expanded Universe, since both the Empire and Republic du jour are generally riven by internal strife and ambitious individuals are all too happy to latch onto any plan more complicated than "go to the next room and get a drink" for their own benefit. The most impressive:
Wedge's Gamble: Wedge Antilles and his crack pilots-slash-infiltration team are told to bring down Coruscant's planetary shields. They intend to do so by inserting a programming hook in the planetary computer system that will let them simply tell the computer to turn them off. This will be simple, they reckon, because a Rebel sympathizer controls the factory that makes hardware for the system. This plan doesn't go to spec, so Wedge ends up furiously improvising a strategy that uses an orbital mirror satellite, a giant floating construction robot, and a bunch of black-market starfighters. It works.
Shatterpoint: Mace Windu intends to sneak into the Confederate-held system of Haruun Kal, and find his renegade ex-Padawan, ostensibly in charge of a local partisan movement, who may have gone a little Heart of Darkness on them. Once he finds her, he'll just call in a Republic cruiser that will punch into the system, send down a shuttle, and jump out; no muss, no fuss. This plan goes to hell in the most direct way imaginable when the whole system is a Separatist trap and Depa's Ax-Crazy associates decide they want the war to continue. Mace ends up winning, of course, but not in the way he would prefer.
Rebel Dream: Perhaps the simplest of them all: all Wedge Antilles (again) has to do is not get killed defending Borleias from the Yuuzhan Vong. ( Well, the Advisory Council would prefer he die, just not right away.) Between his stubborn refusal to listen to a stupid order and his particular brand of tactical genius, he manages to not only hold the system against the unstoppable alien horde for months and withdraw in good order, rather than being overrun, he sets up a quasi-independent resistance movement to take over when or if the Republic collapses and manages to kill one of the top Vong generals in the bargain.
Live Action TV
This is what Frasier generally does when it's not doing Threes Company. Lampshaded at one point when Martin scoffs at yet another Simple Plan that Niles and Frasier have devised (sharing an office as psychiatrists):
Martin: The book you two tried to write together; that was a bad idea. The restaurant you opened together; that was a bad idea. But this... no, that restaurant was still the dumbest.
As indicated above, it also happened frequently in Fawlty Towers.
Frequently occurs, and is lampshaded, in Farscape. As Aeryn puts it to John "Your plans never work! Not the way you detail them!" Eventually he admits this when an unspoken plan actually works as planned, "Yeah, it's a first, isn't it?"
There is even an episode subtitled "A Not So Simple Plan".
Played painfully straight on the My So-Called Life episode "Life of Brian" where the simple plan is to go to the school dance. One kid changes dates, which sets off a chain reaction of people changing theirs, until nobody is happy with what results. Despite sounding comedic, the episode showed what it would be like to actually have to deal with that in real life.
The collapse of "simple plans" was a common plot point in Firefly, although usually due more to bad luck than anything else. This led Mal to gripe in the episode "Safe," upon finding himself in yet another shootout, "Why don't it ever go smooth?"
He suffers this to the point where it's made into a character trait ("Things Don't Go Smooth") in the Serenity RPG. Mal has the severe version of this trait. Player-made characters can take this drawback as well, giving the Game Master an excuse to ensure that Hilarity Ensues no matter how well the players plan their various hijinks.
Many an episode of Drake & Josh uses this trope, memorably one where their attempt to deliver a cake to their elderly aunt's wedding leaves them stranded in the middle of nowhere and blowing up the car they borrowed.
A somewhat common theme in Leverage. Of course there's a reason it takes five of the world's best thieves to pull these cons off. Even the simplest plans have potential to go all to hell for reasons as simple and unpredictable as a mook of the bad guy calling his cousin or as large as someone trying to crash land the plane they're on to run a con.
Lampshaded to a degree in The Mentalist, a drama. Patrick Jane refers to one of his schemes as being a "simple plan" and Lisbon says that she likes it simple. It's when he starts getting out the costumes and such that she gets worried.
On one Friends episode, the plan is to throw Rachel a birthday party. Things start getting messy when the parents refuse to be in the same room and two parties get planned and then...
Happens all the time in Stargate SG-1, nicely lampshaded in the episode "Off the Grid" after a plan has gone pretty badly wrong:
Jackson: Uhh…I have a question: Why would we make the Gate magically disappear BEFORE we had a chance to escape through it?
Worrel: Bad timing?
Jackson: Th-that's got to be the single stupidest thing I've ever heard.
Worrel: Do things always go according to plan in your world Dr Jackson?
Doctor Who. Anytime the TARDIS lands and the crew has something relaxing or normal planned—going to the beach, visiting their family, maybe just picking up some milk—will end up with them all running for their lives. Every. Single. Time.
In "The Doctor's Wife", the TARDIS, temporarily put in a human body, says that she doesn't bring the Doctor to where he wants to go, but where he needs to go.
The MythBusters have often come across myths that, on the surface, seem easy enough to test...and then some factor they hadn't counted on makes it a nightmare.
Adam: You should never expect Plan A to go off without a hitch! Me and Jamie, it's usually Plan D.
In Season 3 of Justified a corrupt prison guard and his prison nurse buddy have the great idea to break Dicky Bennett out of prison, have him lead them to the $3 million his late mother hid away and then kill him. They do not count on the fact that they will have to take Dewey Crowe with them or that US Marshal Raylan Givens will suspect them right away. They also do not realize that most of the money is already gone or that the person holding the rest of the money feels honor bound to keep Dickie alive and is much deadlier than they are.
Happens often in the series Sanford and Son when not a Zany Scheme. In an example, one episode begins with Lamont bringing in an old Revolutionary War rifle to restore and quickly leaves the title characters wondering if they have accidentally murdered a neighbor.
Two words. (Or, rather, one word, twice:) 'Allo 'Allo!. Used quite often. Also Lampshaded when René says "Simple plans are always the best!"
The RPG Exalted has a dramatic example: as penalty for their actions in overthrowing the Primordials, the Sidereal Exalted, Heaven's bureaucrats, were cursed so that whenever they get together in large numbers to create a plan, it usually goes horribly, horribly wrong. How horribly wrong? Well, seeing as their last great get together resulted in the death of Creation's god-kings (mind you, they kind of deserved it) and could be indirectly blamed for an apocalyptic disease, an invasion by The Fair Folk, the fall of a Golden Age of magic and technology, and untold amounts of Creation's landmass dissolving back into the Wyld, they can only go up from here.
And that's not even mentioning how the rebellion against said god-kings broke a constellation.
This sort of nonsense is de rigueur in Paranoia, but occasionally, you can get a subversion.
The introduction of the Shadowrun Fourth Edition rulebook has an experienced runner asserting that the scariest words in the world are not "trust me" (since many people in this Crapsack World learn young not to trust others), but "It'll be easy." Then goes on to detail a supposedly easy corporate espionage and sabotage mission that, due to a double-cross by the Johnson's Humanis-connected assistant, ended with two of the narrator's team being fatally shot, as well as the team almost being responsible for the mass murder of a sizable amount of the ork and troll population because of the assistant's poisoning of the stuff that was supposed to go into the rival company's drink to make it taste bad.
Le Bret and Ragueneau plan to leave Roxane unaware of Cyrano’s mortal wound when they left the nunnery to attend him.
The Playstation game Incredible Crisis revolves around a day in the life of the Tanamatsuri family. The Simple Plan is to get home in time for grandmother Hatsu's birthday party, but the other members of the family get involved in one bizarre misadventure after another. To name just a few: Taneo, the father and a grade-A Salaryman, gets chased around by a giant globe that fell off a statue, and at one point ends up going the wrong way down a busy highway on a stretcher; Etsuko, the mother, gets kidnapped by robbers and eventually ends up piloting a jet fighter into battle against a giant rampaging teddy bear; Tsuyoshi, the son, gets shrunk by weird alien rays, and has to dodge insects while trying to make his way home safely; and Ririka, the daughter, has to protect a tiny UFO while helping it reunite with the "mother" ship.
There is of course Conkers Bad Fur Day where Conker's plan of simply going home after a night of drinking doesn't work at all.
The real-life development of the game is arguably an example as well, since the development team started out with the goal of making a generic cute-animal 3D platformer. Quite a lot of delays later, it didn't quite turn out that way.
The main plot of Grim Fandango is the result of two Simple Plans colliding:
Hector Le Mans: Defraud deserving souls of their golden tickets, sell fake tickets to rich people, (un)live in luxury actually keep all the golden tickets to balance out his many crimes and get him to the Ninth Underworld.
Manuel Calavera: Steal a colleague's work order in order to break a slump and maybe work off his time in the Land of the Dead a little faster.
About ten minutes after Calavera's plan is put into action, a chain of events is touched off that completely reshapes the underworld.
Combined with a massive dose of Genre Savviness in Sluggy Freelance. After a relatively simple plan to steal a Mad Scientist's Displacement Drive Vehicle goes awry and ends up destroying the vehicle and scientist's entire island complex, Torg learns the experience. So, when he decides to try and take down Hereti-Corp he figures the best way is to pretend that they have a Displacement Drive Vehicle, too, try and steal it, and cause so much unintentional pandemonium that it brings the whole company down.
"I just wanted a Good plan! Torg, that's nigh-foolproof!"
The tendency for simple plans to go horrible wrong is lampshaded in thisWapsi Square strip. Said simple plan results in being shot at and bringing down a group of smugglers in Cairo.
Happens with such frequency in Legostar Galactica that Captain Smith has started viewing it as little more than a mild, entirely routine annoyance, sighing "of course it is" whenever he's informed of some new, bizarre, unpredictable development
Most Strong Bad Emails are like this. A simple e-mail asking if he's ever been on a road trip leads to him and The Cheat being stuck in the car all day, and a pretend pizza place to meet some girls results in an actual, well-reviewed pizza place.
Anika just wanted to get some water for her parents. Then an eagle mistook her toy for lunch.
Somewhat subverted in the Jem episode, "Broadway Magic" in which using a forged letter from "Rio", the Misfits are able to lock Jem in the crown of the Statue of Liberty. However, the Misfits are unaware that Jem's earrings are part of a illusion-creating computer. The show plays this trope straight in many episodes.
"Everyone has a plan, until they get punched in the face." —Mike Tyson.
Paraphrasing Helmuth von Moltke the Elder: "No plan of operation survives contact with the enemy."
The Triple Alliance's plan for World War One: in case of war with the Entente, the Austro-Hungarians go all-out on the Eastern border against Russia while Germany holds on their own border, in the meantime most of the Germans and the whole Italian army invade France on two fronts (with the Germans passing from Belgium), ensuring that one or both forces will reach Paris and knock France out the war, and then everyone gangs up on Russia, forcing Britain to ask for peace, and then everyone's Home by Christmas. Italy declared neutrality (and would later enter the war on the Entente's side), leading to France holding the line on the Marne and the Austro-Hungarian offensive being beaten back when the Russians managed to mobilize enough troops.