Mistaken for Dying

One character mistakenly thinks that another is terminally ill. They may feel like they have to be especially nice to the "dying" person, or they may attempt to "cure" them somehow.

Usually the supposedly dying character has no idea what is going on. The other character(s) may avoid telling them because just talking about the terminal illness might kill the poor victim from "shock". Alternatively, the "dying" character might take advantage of the situation, leading to an inevitable comeuppance once his friends discover the truth.

Another example is when the focus character mistakenly believes himself to be dying. As a result, he decides to take suicidally risky dares in order to go out in a blaze of glory (and, in some cases, to avoid the ravages of the imagined disease). Often times, the other characters tell him in the midst of one of the dares that he is in perfect health. His flash of relief is immediately replaced with mortal terror as he realizes that he has unnecessarily placed himself in mortal danger and has to be rescued.

Compare Hypochondria and Not Now, We're Too Busy Crying Over You!.

Examples of Mistaken for Dying include:

Anime & Manga

A well played/comical version of this was on Joshikousei when the "Dying" person thought her friends were helping her audition for a role and played along with it! This was subject to a Snap Back at the end with a spot of Lampshade Hanging in the next episode: "Weren't you dying of a terminal illness?"
Manga subversion: The manga version of Excel Saga had Iwata (a subversion of the Sentai Hero) go through this trope in one chapter, which ended by revealing that his doctor (and Sitcom Arch Nemesis) had faked his diagnosis. The next chapter started with the team's mentor Kabapu announcing that Iwata had died; the doctor had only exaggerated the symptoms to try to get the case into a medical journal. (Of course, Iwata was only mostly dead; they save his brain and turn him into an Artificial Human—after sticking it into the body of the Robot Girl for a few chapters...)
A central theme of one episode of Macross 7, though in this case the "victim", Miriya, is convinced that she's dying due to a diagnosis from Doctor Chiba, and spends the episode completing her "last objectives". In the end, it turns out she has the flu. Though why she believed the medical diagnosis of a doctor whose primary specialty is giant speakers mounted on jet fighters...
This was a plot of one episode of Inuyasha, when the group believed Miroku's teacher when he said was about to die, after one of the characters saw him coughing up blood (he was actually choking on a sharp fishbone, not suffering from a terminal illness). They only learn at the end that he simply thought he was going to be the next in a string of murdered monks. Of course, he had nothing to worry about, but this didn't stop him from getting his house cleaned up, dinner at an expensive inn, and sake from a deathtrap mountain, and so on, as his dying wishes.
Used as a filler episode in One Piece. The old guy was diagnosed with a fatal heart condition by the crew's resident doctor and everyone joined in to make his last days more comfortable. He ended up having so much fun that he got better, somehow... Although the end of the mini-arc strongly implies that he died soon after helping the Strawhats break through a Marine blockade.
This happens with Gloria in episode 9 of Daphne in the Brilliant Blue - after a group check-up the cast believes she has advanced terminal cancer that will kill her in a week, but in the end it turns out to have been caused by a series of coincidences.
One episode of Seto no Hanayome has both types of this trope. Kai tries to hide the fact that he has a pimple on his butt, and out-of-context dialogue from his doctor leads to his friends assuming that he's been stricken with a terminal illness. All of his friends then proceed to act nice to him, confusing him until they drop the bomb that they know he's going to die. Amusingly enough, he actually is convinced that he will die, and this enables him to briefly get over his debilitating phobia of wide-open areas to have a final man-to-man showdown with Nagasumi and seemingly die happily. And then everything goes back to normal the next day.
In an apt (IMO) inversion, Tai thinks he can't die while in the digital world because it's not really him. After crossing a deadly barrier without worry, it gets explained to him that he can, leading to his freak out when trying to get back out.
In a less deadly showing of this trope, a filler episode of Hayate the Combat Butler, the characters think Yukiji is going to quit as a teacher and they make an effort to show that they're interested in keeping her around. Played with since the note that'd been found was actually Makimura's given to Yukiji as a carrier, and she eventually decided to stick around.

Comic Books

In DC Comics, a doctor fakes a medical report to con the Joker into believing that he's dying. The doctor hopes to "scare him straight". Instead, the Joker goes on a final rampage of such hideous scope that it becomes a Crisis Crossover.

Films -- Live-Action

Short Time, a forgotten 1990 comedy with Dabney Coleman and Matt Frewer, hinges on an occurrence of this trope. Coleman plays a risk-conscious cop who has the results of his physical switched with another patient and is told he is dying. Coleman wants his son to go to college so he plans to get himself killed in the line of duty and his family can collect on insurance. But he only has three days left before retirement, so he has to work fast.
Another example comes from the 1990 Tom Hanks film Joe Versus the Volcano, where the titular Joe is willing to be sacrificed to a volcano god because he thinks he's got a fatal "Brain Cloud."
- This inspired the 1991 Teenage Mutant Ninja Turtles episode "Raphael vs. The Volcano," where a misunderstood overheard conversation leads Raphael to believe he's dying. He takes on especially dangerous crimefighting endeavors without the other Turtles since (he thinks) he has nothing to lose.
More recently, the 2006 film Last Holiday starring Queen Latifah, which in turn is a remake of the 1950 film starring Sir Alec Guinness.
Can't forget this bit from Monty Python and the Holy Grail:

"Oh sweet Concord, you shall not have died in vain!"
"Um, I not quite dead Sir..."
"Well... Then you shall not have been mortally wounded in vain!"
"I think I'm going to pull though, Sir."

- This is a Running Gag throughout the film, brazenly subverted in the Castle in the Swamp scene when Prince Herbert's father orders the bride's father killed after he shows signs of "getting better".
A variation in Almost Famous: it appears that the band's plane will crash, resulting in an outpouring of everyone's emotions and secrets. The last guy to talk simply shouts "I'm gay!" Then the plane stops shaking.
In Killing Emmett Young (also known as Emmett's Mark), Emmett is given a false diagnosis of a fatal illness, which causes him to put more effort into his work as a detective, as he figures that this will be his last case.
Happens less than you might think on On the Buses. One example is in Bye Bye Blakey, where Stan overhears Blakey discussing his plans to leave his job, and believes he is dying instead. As you do in these situations in Sitcomland.
Happens in Eat Drink Man Woman, when Jia-Chien sees her father coming out of the hospital's cardiac department and thinks he has heart problems.
In the Polish comedy Zemsta!, the main character Papkin becomes convinced that he is dying. None of the other characters express any sympathy or bother to tell him that he's wrong.
"I'm not dying, you idiot!"
A classic example from Up:

Russel: *pokes Carl at least twice. Carl moves* Whew, I thought you were dead!

In Lethal Weapon 2, Riggs is shot multiple times. After taking care of the Big Bad, Murtaugh goes to where Riggs is lying in a pool of his own blood, complete with "Knockin' on Heaven's Door" playing in the background. The following conversation ensues:

Murtaugh: You are not dead until I tell you! You got that, Riggs?
Riggs: Hey, Rog, *cough* could you reach into my pocket there?
Murtaugh: *pulls out a pack of cigarrettes and offers one to Riggs* Here you go.
Riggs: I... I want you to throw those things away; those things will kill you.

Literature

In the story The Adventure of the Dying Detective (in the collection His Last Bow), Sherlock Holmes fakes having a terminal disease in order to lure the villain into a trap, but because he doesn't trust anyone's acting ability but his own, he deceives not only the villain but everyone else as well.
During a later part of The Belgariad, Garion's cousin Adara gets shot with an arrow; on her deathbed, she reveals to prince Hettar how she always loved him and now they'll never be together... until he points out that she was actually only lightly wounded and isn't going to die at all. She's understandably embarrassed and wants to run away and hide, but Hettar's having none of that. Ah, love...
In L. M. Montgomery's The Blue Castle, the protagonist is told incorrectly that she has a fatal heart condition, and likely won't live a year. This leads to her saying what she thinks without regard for what her (overly judgemental) family thinks of her, taking a job caring for a "ruined", dying girl, and marrying the town's mysterious ne'er-do-well.
A similar example to The Blue Castle is Rita Mae Brown's Venus Envy. The heroine, thinking that she's dying of lung cancer, writes letters to everyone she knows telling them exactly what she thinks of them (which, in many cases, is not nice). Furthermore, she comes out to everyone who didn't already know. Turns out that it was pneumonia, not lung cancer...and, by the time everyone's gotten their letters, she's recovering.
In Millicent Min, Girl Genius, the main character overhears her mother throwing up in the bathroom, regularly, and other similar symptoms. Being a genius, she deduces that her mother has terminal brain cancer, and does not share her findings with anyone - and (happily) eats humble pie when it turns out her mother is actually pregnant.
In Blood Pact, Colonel-Commissar Gaunt is convinced Ayatani Zweil is dying of blood cancer. In reality, it's Tolin Dorden who is dying. Zweil just switched the labels on their test samples out of mischief, because he was convinced both of them were perfectly healthy and didn't need to waste time on routine medical exams.
At the end of "Fool Moon" Harry Dresden, believing that Murphy shot him, forgives her generously and is well into the "I'm Cold... So Cold..." routine before she disillusions him. She was shooting at the man behind him, and there's a cold front coming.

Live-Action TV

Hey Dude also did the die-if-he-goes-to-sleep variant, with the cast accidentally overhearing a conversation with a doctor - a doctor that turned out to be a veterinarian diagnosing a horse (which also led to them throwing a party with mashed carrots and oats in the snack bowls).
In Yes Minister, Sir Humphrey describes his promotion in a way that gives Hacker the impression that he's dying.
On Frasier, Daphne thinks Martin is dying, when he's just starring in a nativity play.
- And in the last episode, everyone thinks Frasier is dying, when he's just moving to San Francisco. Done quite well; rather than an offhand remark everybody misconstrues, it begins when Frasier claims he went to the doctor to cover for missing a day's work, then later goes to the doctor (without telling anyone why, as he's embarrassed) to have botox, but the procedure gets mixed up and makes his eyes water constantly like he's crying. He gathers his friends (to tell them he's moving, but they don't know that) and gives some prized possessions away. Even then they are reluctant to jump to the obvious conclusion, but a message gets left on Frasier's answering machine that they all can hear which speaks of his medical results (from the Botox, but they don't know that) "not looking good." Followed by Frasier announcing his move to San Francisco in this way, "We all knew we weren't going to stay together forever. I mean, that's not what life is about. Cry, if you must, but I assure you, when I pass through that Golden Gate I will be smiling!"
Parodied on Newhart, when Dick stars as the father in Stephanie's short-lived sitcom. Stephanie (playing twins) overhears him saying: "Tonight, I am going to die...my hair."
Happened in The Adventures of Lano and Woodley, in the episode "One Simple Task". Col and Frank were planning a trip, but Frank forgot to book a spot at the caravan park. Frank invents all sorts of outlandish stalling methods while he frantically tries to get a spot. During one conversation, the caravan park tells Frank they'd have a spot in a week, causing Frank to protest that "in a week he'd be dead". Cue Col, who overheard the last part.
On Its Always Sunny in Philadelphia, Charlie tells Dennis he has cancer, hoping that the news will make its way to the waitress he has a crush on, who he has seen wearing a Livestrong bracelet. Unfortunately for Charlie, she instead feels sorry for Dennis because his friend is dying, and they end up sleeping together.
Penultimate episode of Star Trek: Voyager gives us the Doc's "final speech": Thinking he's dying, he apologizes in excruciating detail for every wrong he's ever done, even those no one knows about. Only of course it turns out he's fine. Yikes.
In the episode "Half-Wit" of House everyone thinks that House is dying of brain cancer, and it actually looks like he is for most of the episode. However, by episode's end, everyone discovers that House was just faking having cancer so he could get high off of the powerful drugs cancer patients are given.
A purposeful mixup of checkup patients in Hogan's Heroes makes Klink think Schultz has a few days to live, when really he's just going on leave for the weekend.

Schultz: "I wish you were going with me!"
Klink: *pained* I probably deserved that..."

An episode of Titus featured Christopher and his father both believing that the other was dying. (The were both lied to by Christopher's fiance so that they would spend time together.)
- An earlier episode had Christopher and his friends thinking his father was already dead, and having an impromptu wake/roast until dear old Dad showed up to berate him in the episode's final minutes.
  - Hilariously, that's the pilot.
Happens to Sixth Ranger Zhane in an episode of Power Rangers in Space. Being frozen for two years messed up his powers, and he can only stay in morph for two and a half minutes—obviously, the other Rangers' eavesdropping didn't last long enough to catch that part - only about it not looking good, and Zhane not having much time. Zhane actually hears them discussing his impending death, and decides to have some fun with it. Unlike the subsequent season, when Andros himself refers to Kendrix's Heroic Sacrifice as if it were of her powers rather than her life, the word 'dying' is used twice in the episode.
Done in The Mitchel And Web Look where in one sketch a man waits at a train station for his girlfriend but coughs "What's the matter dear?" "Oh, it's just a cough". Repeat around three times, gradually getting more weak and feeble, then one day he's not at the station! Oh no he's just a bit late and he feels much better but the girl was only going out with him because she thought he had pneumonia.
In Star Trek: Deep Space Nine episode Body Parts, Quark thought that he had a fatal disease from a doctor on his homeworld so like all Ferengi, he arranged to sell his remains. It turns out that the doctor was wrong.
- The really funny part of that one was Quark's reaction to the misdiagnosis: "This means I get to sue the doctor for malpractice!... And I'm going to live!"
A Sanford and Son episode has the "being extra nice to the 'dying' person" version when Lamont overhears Fred's doctor telling him he "has six months left" (to pay his bill) and jumps to the wrong conclusion.
On Who's The Boss?, Tony's father-in-law is trying to admit to Tony that he is going to jail, but he can't bring himself to say the word jail. Tony thinks that the unspeakable truth that his father-in-law can't say is that he's going to die (because after all, if he were dying, it would be hard to say that as well). The father-in-law spends the episode milking this undeserved sympathy, because after all, this is much preferable to admitting that he's a convict.
In Childrens Hospital, Lola wants to break up with Owen, so she tells him she has a brain tumor. He freaks out and vows to find a cure. Realizing what went wrong with her "cunning plan" Lola tries too tell him the truth, but Owen insists that every denial is "just the brain tumor talking."
Stargate Atlantis: Rodney McKay is immune to a nanite virus, but neither he nor those he's quarantined with know that.

Newspaper Comics

A Gil Thorp plotline from several years ago involved student Brynna thinking she'd overheard that her coach had breast cancer. Without checking on whether or not it was true, she wound up shaving her head as a show of support...only to learn this was not the case and embarrassing herself. However, this did lead to the other girls on the team ended up either shaving their heads or simply cropping their hair extremely short to support breast cancer research.

Theater

This is the entire premise and plot of the play Send Me No Flowers.

Western Animation

Best example: that episode of The Flintstones where everyone thinks that Fred will die if he goes to sleep, so they construct an elaborate plan to keep him awake for the next seventy-two hours.
The Jetsons did this one, in the episode where George thinks he's only got a few weeks to live, and in the process of telling off his boss ends up being volunteered to test an experimental indestructible suit.
The Simpsons also did this in "One Fish, Two Fish, Blowfish, Blue Fish", where Homer is told he may only have twenty-four hours to live after eating badly prepared fugu sushi. Especially interesting for two reasons: first, that there is no mistake, just an even chance of life or death, and second, that this is given the dramatic weight it deserves.
- A Halloween episode (about Y2K destroying technology all over the world) where they run into Krusty in agony because his pacemaker is hurting him. Homer believes he had died but Krusty yells "I'm not dead!" as they're walking away.
- Hans Moleman gets mistaken for dead, twice.
Rebecca and Baloo both thought he was dying in the Tale Spin episode "Barely Alive" when a mechanic's report that an airplane part was broken beyond repair was misinterpreted as the results of a doctor's physical. Humorous mostly because Kit hears the news, knows what it's in reference to, and fesses up that he broke the part -- "I didn't mean to! I was just playing around with it and it fell out! Maybe I can get another one at the junkyard..."—much to Becky's initial horror and confusion.
The "mechanic's report mistaken for doctor's diagnosis" premise was recycled in the DuckTales (1987) episode "Scrooge's Last Adventure". Huey, Duey & Louey took Scrooge McDuck's grandfather clock to a mechanic after they accidentally broke it while playing inside. While at the same time, Scrooge was getting a checkup at the clinic. From there it turned into a parody of Tron.
Another 'Talking about a machine' incident happens in Donkey Kong Country, with local Buffoon Bluster thinking he's going to die. He resolves to be a good ape for the remainder of what little time he has left. We Want Our Jerk Back insues...
The Johnny Bravo episode "Carl Be Not Proud" had Johnny think a report that Carl's sickly venus fly-trap had only twenty-four hours to live was talking about Carl, and Carl taking advantage of the situation by talking Johnny into doing his bidding for the day.
An episode of Top Cat had Officer Dibble mishear a conversation between T.C. and a doctor about a broken clock and think T.C. only has one week to live. After a few more misunderstandings, T.C. figured out the situation and tried to take advantage of it. Of course Dibble eventually found out and got quite angry at this development.
Camp Lazlo, "The Big Cheese": Scoutmaster Lumpus thinks he's going to disintegrate after eating a certain kind of rare cheese.
SpongeBob SquarePants, "Dying for Pie": Squidward thinks Spongebob has eaten an exploding pie, and does everything he wants for a day.
A variant occurs in the Ed, Edd 'n' Eddy episode "A Case of Ed", where Ed and Eddy trick Edd into thinking he has a rare and deadly disease.
Also happened in the Godzilla: The Series, where Randy reads an email to Mendel, saying that "his virus is fatal". Little did he know that the email was about a mummy Mendel examinated beforehand.
Another example of "mechanic's report mistaken for doctor's diagnosis" occurs in an episode of The Raccoons, where Cyril Sneer is ill and his pig servants, overhearing the doctor on the phone about his clapped-out car, mistakenly think Cyril "only has a few days to live". At first, Cyril also believes this "diagnosis", but, on learning that he only has a cold, keeps up the pretence in an attempt to force the Evergreen Standard(which needs a new printing press) out of business. However, Bert Raccoon thinks of a way he and his friends can both commemorate Cyril and save their paper - a memorial printing press.
South Park episode The Death of Eric Cartman combines "mechanic's diagnosis mistaken for doctor's diagnosis" with the other kids coincidentally getting so angry with him they decide to ostracize him. The result is Cartman believing himself to be already dead.
Drawn Together has an episode where Captain Hero hits his arch nemesis with a car, and decides to take care of him on his death bed. However, it's just an elaborate ploy to get CH to wash his balls, a recurring theme between the two.
Pickles thinks he's dying in an episode of Metalocalypse. It turns out everyone in the band switched their medical records with each other and the real one dying was Toki's cat.
Variation/subversion in an episode of Recess. After the kids find a cootie-catcher and discover that the predictions made with it are coming true, Gus believes he will die the next day, as predicted. So, he decides that if he's going to die, he might as well do everything he was too afraid to do before...including trapping and humiliating the bully he attacks him all the time. After the good fortune everyone had runs out and they realize Gus won't die, they arrive to tell him this just as the bully is about to get trapped in a net. There is a subversion however, as Gus doesn't regret doing what he did, and the bully becomes terrified because he realizes that Gus nearly pulled one over on him and wonders who else might do that.
In one episode of Goof Troop ("Terminal Pete"), Pete believes that he only has a few hours to live after hearing from the doctor about his "terminal illness", and so he needs to spend the last hours of his life playing stuntman. However, the "terminal illness" thing turns out to be a joke, made up by the doctor and played out when Pistol's gopher had snuck into the sheets where Pete was lying on when the X-ray picture was taken.

Web Comics

Ellen from El Goonish Shive thinks she's doomed to be as short-term as the Applied Phlebotinum that caused her storyline. Initially, she tries to be an Evil Twin to leave her mark; she then tries the blaze of glory: 'Sorry, but it's kind of hard to frighten the dead' spoken deadpan to a hideous blob monster.