Windmill Crusader

This character would have been a hero. He’s fighting the good fight. His goals and methods are reasonable, considering the circumstances. There’s only one problem: The circumstances are not real.

Since the premise for his actions is incorrect, his actions are not reasonable either. In fact, his actions can be quite outrageous when he does what he has to do to save people from a threat that doesn’t even exist.

A Windmill Crusader is not a Straw Hypocrite: While he might be a Heteronormative Crusader or Principles Zealot, he is not a dishonest person who will try to trick people into a struggle against windmills for the greater good. No, he honestly and wholeheartedly believes the windmills are giants, and is desperately trying to save everyone from this nonsense threat.

So, why did he mistake windmills for giants in the first place? Well... maybe he's crazy, maybe someone gave him misinformation that he didn't check up enough, or maybe he blindly believes some religious or ideological dogma.

Fighting a threat that others mistake for a windmill but actually is a real threat doesn't make the character a Windmill Crusader at all. However, it makes it easy to get mistaken for one. Also, a character can fight a real threat and still be a Windmill Crusader: either by misunderstanding what the threat really is, or by fighting real threats as well as windmills.

Might suffer from Black and White Insanity or Aggressive Categorism, and is likely to do Activist Fundamentalist Antics.

Compare Conspiracy Theorist. Contrast Lord Error-Prone for someone who makes a lot of little mistakes rather than one huge fundamental misconception.

No Real Life Examples, Please, as people are bound to disagree as to whether or not various controversial philosophers, activists, political leaders, or religious prophets fall under Windmill Crusader, Straw Hypocrite, The Cuckoolander Was Right or even the Only Sane Man.

Anime

 * Martian Successor Nadesico: A major theme, especially the latter half of the series. Amusingly, one could say that Gai Daigouji basically is Don Quixote, only this time some of the windmills really are giants. Naturally, he doesn't last very long.
 * Ranma ½: Tatewaki Kuno carries elements in this, mainly when he attacks Ranma, believing that he has enslaved Akane and the 'Pig-Tailed Girl' First problem with this is that Ranma and the 'Pig-Tailed Girl' are in fact the same person.
 * Working: Inami was taught from a young age by her father that males would all (the 'windmill'). By the time the story rolls around, she has a severe fear of men because of this
 * The utterly insane and Obliviously Evil inquisitor Mozgus in Berserk. His list of activities include: self-mortification by slamming his body into the ground thousands of times a day during prayer, using his sacred book to bash the brains of any "heretic" he comes across (in addition to bludeoning them on breaking wheels), and torturing the rebellions of the poor and starving population (including a young mother) - all under the desire to make those he considers sinful (which means everyone) to repent (by torture) so that their souls will be saved.
 * The Principal from Angel Densetsu believes that the main character, Kitano, who is being threated as though he is the demon king himself, going as far as to bring people into the school, with the sole purpose of beating up Kitano, or better yet, get him expelled. Of course, Kitano is anything but evil - He merely looks evil.

Comic Books

 * Dilbert features many surreal jokes based on the premise that one character lives in his own little reality. Sadly, this is often a character who has power - or who gains power by enforcing her crazy perceptions on others.
 * In Lucifer, we have the political faction "Efferul for Lucifer" that fights on the Morningstar's behalf. He is not amused, as their agenda is based on a very misguided vision of what he wants and needs.
 * Bitchy Butch: Bitchy is infamous for this among her fellow gay rights activists. She's paranoid about men (oppressors all of them!) and heterosexual women (traitors!) as well as lesbians (potential traitors, "not real", or whatever), and see the religious right in every shadow.
 * Quantum and Woody: Played straight for drama. Quantum is convinced that David Warrant engineered the deaths of his and Woody's fathers, and tried to kill them in the accident that gave them their powers. He still suspects this even after repeated non-violent encounters with Warrant, including one time when Warrant helped save Woody's life. It reaches cataclysmic proportions when

Film

 * In Downfall, Hitler and some of his closest followers are portrayed as tragic antiheroes who desperately tried to save the world from a world-engulfing conspiracy that they honestly believed to be real. As Berlin falls they face what they believe to be the twilight of mankind itself. Hitler himself is portrayed as a person who is most likely insane, while his followers are rational except for their misguided belief that he is a legitimate leader rather than a madman. Their actions make total sense when one take this tragic belief into account.
 * In A Beautiful Mind, the protagonist is the Real Life Nobel Prize winner John Nash, hired by the US government in their struggle against terrorism. What neither Nash nor his closest superiors know
 * One minor character in Terminator 2 and Terminator 3 is a psychiatrist who is so convinced that Sarah Connor is a Windmill Crusader that he becomes a Windmill Crusader himself. The time-travelling robots are really No Mere Windmill, but in his refusal to believe this reality he clings on to a Windmill Political about hallucinations.
 * The Last Temptation of Christ
 * Jesus is initially portrayed as the insane kind of Windmill Crusader. This is played straight for most of the movie, he even gets cured of his messiah complex and gets to live a normal life. Later events radically change the picture.
 * Paul is briefly portrayed as the misguided kind of Windmill Crusader. However, he is quickly deconstructed as a Straw Hypocrite who simply doesn’t care if the gospel he preaches is true or not.
 * Shutter Island:.
 * Defendor: Played with, and maybe averted.
 * Where Eagles Dare: Played with/inverted; all of the team are sent in to the castle to rescue the general, but only Smith and Mary are aware that the whole setup is a complete fabrication, and most of the rest of the team are the bad guys.
 * A reference to the Trope Namer, the Star Wars comics from Marvel had a minor hero named Don-Wan Kihotay.
 * Arguably, the Operative from Serenity. He'll do anything to create an Alliance without sin, mistaking himself for being a Soulsaving Crusader... until Mal and crew give him a front-row seat to what such a thing looks like.
 * God Bless America: Frank, who seem to believe that he's actually doing something worthwhile. Unlike Roxy, he seem rather naive - not unlike the retarded television kid he keeps identifying with and feeling sorry for for all the wrong reasons.
 * In Bolt, the title character initially believes that the TV show he stars in is real and that his owner has been abducted by its Mad Scientist villain (whose minions are cats, naturally). In reality the movie has no real villain at all.
 * The main character in Creation of the Humanoids is a high-ranking member of a radical anti-robot organization. Recently, they've finally found evidence that the robots actually are engaging in a conspiracy of some sort - have the robots finally Turned Against Their Masters, just like the main character's organization has been warning against? Well, no. The robots really are up to something, but it's nothing that anyone has to be afraid of.

Literature

 * Don Quixote: Quixote is the Trope Namer as well as an extreme example. The protagonist really got reality wrong on a very basic level.
 * In the YA novel The King Of Dragons, the protagonist's father is this. His severe PTSD from military service causes him to believe that terrible things will happen if he and his son are found by the authorities, so he gives him Survival Training From Hell. At the end of the book, he is recovering, and tells his son "I mistook molehills for mountains, but I taught you how to climb mountains."
 * In the Ace Diamond novels, a cab driver suffers a nervous breakdown when his wife destroys his pulp collection and he starts to think he his pulp PI named Ace Diamond and that all fictional Private Investigators are real and friends of his.
 * In Going Bovine, there is a constant doubt on whether Cameron is really traveling across the country and saving the world or simply hallucinating due to his mad cow disease.

Live Action TV

 * Buffy the Vampire Slayer: While Buffy has The Cuckoolander Was Right as an inherent trait, the episode “Normal Again” subverts this when Buffy is drugged and hallucinates that she’s been insane all along and that Sunnydale is only in her mind. In this hallucination Buffy is an insane Windmill Crusader before the series started, and has been locked in a mental institution throughout the whole series.
 * An episode of Smallville has a similar premise, with Clark being given a hallucination in which he is a mental patient who only thinks he is a super-powered alien. Specifically, in an inversion of an incident in the pilot, he learns that Lex lost both his legs in a car accident after Clark jumped out in front of him insisting he was saving him.
 * In Inside Scoop they try to pin society's problems on left-handed people, and propose that we should "ban left marriage". (It's a parody of homophobes, of course.)
 * Grotesco 2: In this Swedish comedy show, some religious leaders take their homophobia a few steps beyond Heteronormative Crusader straoight into windmill territory. A protestant, a catholic, a muslim and a jew all agree that it's not their fault they have been murdering each other for millennia—nope, the gays made them do it! Oh, and it's not Satan's fault either: He's also an innocent victim of the horrible gay conspiracy.
 * Degrassi the Next Generation gives us Emma Nelson. It's not enough to boycott genetically modified foods, or even petition to have them removed. She's convinced the lunchlady's trying to poison them.

Tabletop Games

 * In Paranoia, The Computer became paranoid about the Commies (who had disappeared long before Alpha Complex was built) when an early malfunction caused it to mistake 1957 civil defense files as being up to date. Some disgruntled citizens soon re-formed the Commies (as best they could figure them out) because The Computer was paranoid about them, after which their role varied from No Mere Windmill to Strawman Political to The Scapegoat (for PURGE, or some other secret society, or just self-serving individuals).
 * The "Wobblies" society also fall under this. At first, every Troubleshooter sent to investigate this potentially dangerous group reported that they couldn't find anything at all (since the Wobblies didn't exist) and were summarily executed for laziness, incompetence and/or collaborating with the Wobblies. Eventually the Troubleshooters had to found the society themselves just to have something to report on.

Video Games

 * Dragon Age Origins: Loghain believes that the Darkspawn are not a threat, but the neighboring country of Orlais (who occupied Fereldan for much of his life, until he and King Maric drove them out for good) are still the primary concern for the defense of the country. Interestingly, he considers the Grey Wardens themselves to be dangerous Windmill Crusaders who might weaken the nation's defense against Orlaisian invasion, though the Darkspawn threat is quite real.
 * In Dragon Age II it's revealed that some nobles of Orlais do want to reconquer Ferelden for... some reason (national pride, boredom, take your pick...) and are in disagreement with their Empress (who actually planned to  in Origins. So Loghain was on the ball about that, but being Properly Paranoid doesn't mean you're right about everything.
 * Word of God and notes found in the "Return To Ostagar" DLC confirm that yes, and yes, . Loghain's biggest error of judgement is that, as Flemeth puts it, "he believes the Darkspawn are merely another army that can be out-manoeuvred".
 * Maximillion of Northshire, a quest giver in World of Warcraft's Un'goro Crater, will take the player's character on a long quest to defeat the "evil dragons" in the area, rescue the "purse" of a "fair maiden" from the hot spring, a second fair maiden from a high place, and rescued a third maiden from a "foul beast." Finally, he takes you to something that is actually dangerous: kill an Azeroth-equivalent Tyrannosaurus Rex....by running away from it while throwing his armor at it. At the end, he's convinced he's truly done good. Also notable is that one of the rewards for his final quest is a trinket called a Toy Windmill.
 * N from Pokémon Black and White truly believes he's rescuing Pokemon from people, but only because

Web Comics

 * In Order of the Stick, Miko starts out as a regular Knight Templar but descends into this trope as she becomes increasingly delusional. In the end she is busy saving the world from imaginary threats and interpreting pretty much anything as signs that the Gods are approving of whatever she is doing - ignoring the very real sign that they have stripped her of her paladin powers.

Western Animation

 * In The Simpsons episode "Much Apu About Nothing" the people of Springfield form an angry mob and demand the town government do something to protect them from "constant bear attacks." All because a total of one bear wandered into town, destroyed one mailbox, and was swiftly and painlessly taken down by Animal Control.
 * South Park has Eric Cartman in general, but especially as The Coon in the Coon Trilogy, where he will maim anyone for his gain (including harming a small child for their candy) and rationalize it as an heroic action.