A Bloody Mess

Won't someone please think of the french fries?!

"It's blood!" said the carpenter.
"It's blood, isn't it?" said a musician.
"Blood!!" screamed Christine. "Blood!!"
It was Agnes's terrible fate to keep her head in a crisis. She sniffed her finger again.
"It's turpentine," said Agnes. "Er. Sorry. Is that wrong?"

—Maskerade

This is when a spillage of some innocent red substance is mistaken for blood. This substance is, more often than not, ketchup, and Hilarity Ensues most of the time. This is, however, occasionally used seriously when someone is trying to fake an injury.

Someone may or may not dip their finger in it and taste it, much to everyone else's horror, and pronounce it as being 'tasty.'

It's also worth noting that ketchup looks absolutely nothing like blood, apart from its colour. Smelling like spaghetti sauce rather than an abattoir is also a bit of a clue.

Compare with Symbolic Blood. Prop blood is Kensington Gore. Not to be confused with the Chunky Salsa Rule, neither with a Fallout trait/perk.

Examples of A Bloody Mess include:

Anime and Manga

In Trigun, bottles of tomato juice were used to fake injuries on several occasions, usually to play dead, but sometimes even to attract women.
Full Metal Panic!? Fumoffu does this with red paint in the Haunted Hospital episode. Since Chidori had just fallen through the floor and bumped her head, and the paint was splashed under her head, it was rather disturbing. Actually it was probably one of the more serious notes in that whole comedy series.
One of Usopp's moves in One Piece is the "Ketchup Star" which he uses (on himself) to make the enemy think he's been fatally injured.
In a flashback episode of Shakugan no Shana, Shana sets a trap for the animated training skeleton Shiro that involves a giant, hidden hole in the ground full of ketchup. Shiro sees red and... doesn't take things so well...
In Eyeshield 21, when Hiruma returns to the field after getting his broken arm taped up, his face looks like this. He had some fun with ketchup.
Played with in Karin. The protagonists wipe a pool of blood and replace it with red paint, leading a vampire hunter believe he's fallen for this trope the first time he has seen that blood.

Comic Books

Used for an in-universe Retcon when Spider-Man was killed by a bomb and fought Thanos. Returned to Earth, he found that what had been blood was now the contents of a smashed jar of tomato sauce.
- Another Spider-Man example: In Adam Troy-Castro's Sinister Six trilogy, Spider-Man manages to fake his death using a plate of lasagna and a shard of the window he was blasted in through. Electro is thoroughly fooled, though that isn't saying much. A bystander also reports on the phone to being a witness to the death of Spider-Man; Either he was hiding under a counter during the set-up or was faking it for Spidey's benefit.

Fan Works

A popular subject for fan art of Mobile Suit Gundam: The Witch from Mercury is crossing the Mood Whiplash final scene of the first cour (where a terrorist is splattered by Aerial's giant hand, leaving nothing but aerosolized blood that covers both the heroine and the love interest she was saving), and the love interest's affinity for tomatoes (her main hobby is growing her deceased mom's custom cultivar) in this manner, providing alternate reasons for the heroine casually extending her blood covered hand to her blood covered bride-to-be.

Film

In Looney Tunes Back in Action, the characters do an homage to the shower scene from Psycho. Turns out it's done with a carrot instead of a knife, and Bugs Bunny is squirting chocolate sauce down the drain.
- Parental Bonus, the blood going down the drain in the original movie WAS chocolate sauce.
This occurs in Despicable Me, when one of the orphans gets trapped in an iron maiden, and a red liquid is seen flowing out the bottom. Turns out one of the spikes punctured her juice box.
A deleted intro in The Incredibles has the Parr family attending a neighborhood picnic. While Bob Parr chops up some steaks, a person screams that Bob has chopped off his fingers. However, it only looks like he chopped off his fingers and he's actually fine. To fake the injury and not reveal his super-powers, Bob squirts some ketchup on his fingers and screams in pain.
In Hot Fuzz, a bullet shatters a jar of bolognese, showering it on one of the Andies' faces. The other Andy shouts in despair, thinking the aforementioned Andy has been hit and is bleeding.
- I can't remember off the top of my head, but wasn't it a knife from one of the "two blokes, and a fuckload of cutlery!" in front of the supermarket deli?
- Also, Danny Butterman is adept at using a packet of ketchup as fake blood, so he can make it seem like he's stabbed out his own eye with a fork or even fake Nicholas Angel's death.
In Sam Raimi's The Quick and the Dead, Sharon Stone's character fakes her own death with red ink. When she comes back to kill the Big Bad, we see the little blind kid smiling and pouring red ink on the ground. It's definitely a Crowning Moment of Awesome.
In Memphis Belle, the bomber takes a near-hit from a flak shell, resulting in several crewman being covered in red liquid. They immediately begin checking each other to see who was hit, until they realize that the shrapnel hit a flask of tomato soup.
Used to great dramatic effect in the "Beat" Takeshi vehicle Hana-Bi (Fireworks).
Spoofed in Thanksgiving, a fictitious movie trailer appearing in Grindhouse. Two detectives are kneeling over what is obviously a decapitated human body. One of them dips his finger into the pool of blood around the neck, tastes it, and grimly concludes, "It's blood."
Inverted in Tropic Thunder. Tugg Speedman picks up the severed head of Damien Cockburn, believing his very real death to have been staged, and the head to be a prop made out of latex and corn syrup (which he attempts to prove with a taste test).
Also inverted in Ginger Snaps, when Ginger tells her dad that the large pool of blood on the kitchen floor is in fact syrup. To make it more convincing, she tastes it and offers him some.
In Operation Petticoat a Japanese plane strafes the Sea Tiger. The Prophet has gone on deck to retrieve his guitar (which is destroyed in the strafing run). He sees red goo on him and yells, "I'm hit." Then he puts his finger in his mouth and realizes it's cranberry sauce.
Inverted in Bloody Birthday during a scene where one of the evil children Debbie murders her older sister she and her friends hide the body, her mother comes in and asks what the red stain is and she lies saying it's her sister's nail polish.
Played with a little in Eternal Sunshine of the Spotless Mind. Jim Carrey uses a ketchup packet to "slit" his throat; the motion is so sudden and Carrey's face so grotesque (and the movie itself so weird) that the audience often jumps, thinking he's committed some act of real violence against himself. Kate Winslet walks in, however, and is not fooled (or even amused) for even a moment.
- Not the first time he's done that trick.
Done with a combination of ketchup and actual blood in Cornered
Happens with a red slushee on a windshield in Pineapple Express. Looks convincing enough that it had to be turned black in the trailers due to the MPAA's no-blood policy for greenband trailers.
Happens in Carry On Behind when an explosion upsets a caravan, causing a bottle of tomato sauce to fall out of a cupboard and splatter its contents over Professor Crump (Kenneth Williams). He thinks he's been wounded and faints. Hilarity Ensues.
Played for Drama on Lost when Hurley is accused of murder due to police seeing burger ketchup on him.

Literature

Discworld, Maskerade, as shown by the page quote.
The first bomb of The Westing Game goes off in a kitchen, and a woman runs out covered in red liquid. A few people freak out, but they realize that it's only tomato sauce. The bomb was small, and only caused a shelf of cans to explode.
In Jo Walton's novel Farthing, a murdered man is found stabbed with a pool of red liquid on his chest, but the police quickly determine that the victim was gassed to death and the liquid is actually lipstick.
In Robert A. Heinlein's To Sail Beyond the Sunset, Maureen Johnson wakes up in a bed with a dead man with no memory of how she got there. After two doctors arrive, one of them puts his finger into the "blood" and licks it. She's a bit upset until the two doctors start discussing what brand of ketchup it must be.
Inverted in Dexter is Delicious: There's an obvious wall full of red stuff quite similar to blood, and Deborah wants it to be paint or animal blood in order for the feds not to take her case; for all her trouble, it seems to be human blood (although not her victim's blood type). She still argues that, if it's so much human blood, she has a murder on her hands, and so it's her case.
In War Game by Anthony Price, a Civil War re-enactment is interrupted when somebody finds a genuine corpse lying in a pool of red. It turns out his neck was broken; the red is from a dye pack he was wearing for his big death scene in the re-enactment, which broke while his killer was hiding the body.

Live-Action TV

In the opening credits of Dexter, ketchup squirting across Dexter's breakfast plate is for a split second meant to make the viewer believe it's blood, at least for the first time watching, thanks to the show's gory subject matter.
- Also done in the opening titles of American Psycho.
Also seen in an episode of Lois and Clark, where Clark uses ketchup as a trick to prove he isn't Superman.
In an episode of Lost, Desmond wakes up in a flashback covered in red paint after being in an implosion.
In The X-Files, Mulder did it in the third season episode "Revelations". The look on Scully's face was priceless before he explained that it was fake blood.
Played with in the first episode of Roswell, where one of the main characters gets shot, another heals her... and then he breaks a bottle of ketchup and pours it over her to hide the real blood.
An episode of Psych has a man beleiving he is haunted. There's a message written for him on the wall written in "blood". Shawn notices it's not blood (IIRC, he identifies it as ketchup) and to prove it he eats some. Gus' resopnse? "Enjoy AIDS..."
- You can't get AIDS just by ingesting blood, you have to have the blood injected into your veins.
  - Can. But you have to have open wounds in your mouth for it to happen, and the chances are considerably higher that you won't be infected.
Used in the second season premier of Leverage to fake death by gunshot. You'd think mobsters would be able to recognize the real thing...
An episode of Arrested Development had Gob, at the behest of his father, planning to use dyed corn syrup as fake blood to fake a narcoleptic stripper's death. Things didn't go so well when Buster drank all of the fake blood, mistaking it for juice.
Corner Gas has Emma call up Brent and speak in a hushed whisper, standing over a large red puddle. Turns out she spilt a can of tomato juice and didn't want Oscar to find out.
Inverted on Community: Jeff notices a red patch on his shirt while playing paintball, and thinks he was shot, only to realize to his relief that "it was just blood".
In the pilot of Eureka, while looking for a "lost" kid, Carter and Allison are searching the crime scene and find a hand print. Carter tastes it and declares "Chocolate". He then identifies what brand, and finds the missing kid.
Played with in the pilot of Desperate Housewives: After Mary Alice Young shoots herself, we cut to a scene where her neighbour Martha Huber seems to be examining her blood with her fingertip. It is then revealed that Martha just spilled juice when she was startled by the gunshot sound.
Used at least once CSI when a suspicious red stain on a suspect's clothes turns out to be red sauce. Which, in itself, turns out to be an important clue.
In the Frasier episode "Daphne Does Dinner", after Niles unwittingly insults Frasier's Special Sauce recipe, Frasier throws the whole pan in his face, causing him to stumble backwards through the kitchen doors into the living room, where a couple dozen guests are being entertained, looking as though someone took a meat cleaver to his jugular:

Guests: Aaaaaahhhh!!
Daphne: No no no! It's alright! Look! *licks sauce off Niles' ear* Mmmmm! Tasty!
Guests: Eeeeewwwwwwwwww!!!

In the Law & Order episode "Extended Family", the detectives think they've found a shirt stained with dried blood in a suspect's apartment. It turns out he has a job making fine chocolate and they've merely found his unwashed work shirt.

Newspaper Comics

Parodied in a The Far Side cartoon, with anthropomorphic bottles of ketchup (yes) are watching a horror movie. When an on-screen bottle of ketchup gets murdered, a bottle in the audience turns to his young son and says "Don't worry, son. They're just actors, and that's not real ketchup."
The "ketchup resembles blood in no way beyond color" bit was lampshaded in a certain FoxTrot strip where Jason accuses Paige of punching him so hard that he's coughing up blood, and "proves" it by presenting his Red Right Hand.

Andy: Jason, that's ketchup.
Jason: It could just be that my blood is naturally thick and zesty.
Andy: Just how many colas did you and Marcus drink today?

Radio

In the Big Finish Doctor Who audio The Chimes of Midnight, it opens up with Charley and the Doctor in the dark, when there's a crash and sudden wet feeling. It turns out to be raspberry jam, but it's a unique example because this is a) on the radio so we can't see it, and in the dark so the characters couldn't see that it was red either.

Recorded and Stand Up Comedy

Bill Cosby does a comedy bit about getting his tonsils removed, and when one of the other children comes back he begins to whine "Why's there ketchup comin' out of his mouth? Please say that's ketchup 'cause we'd hate to think that you killed Johnson."

Video Games

In Metal Gear Solid, when you're captured and imprisoned by the bad guys, one way to make good your escape is to use a ketchup-bottle to fake your death, causing the idiotic guard to run into your cell to investigate...
- Another Metal Gear Solid example: The last Mystery mission in VR Missions has you going into a locked room where there's an apparently dead soldier on the ground. There's "clues" all over the place to look at but no suspects. The real goal of the mission is to not give up in frustration at being unable to solve the problem, because just before the time limit runs out, the guy gets up and the camera zooms in on a broken ketchup bottle on which he'd fallen asleep.
In Phoenix Wright: Ace Attorney: Trials and Tribulations, a ketchup stain on a white apron is first mistaken by everyone but the partially color-blind prosecutor for blood.
- In Ace Attorney Investigations Turnabout Airlines Rhoda Teniero mistakes ~~red wine~~ grape juice for blood on a wallet Edgeworth was holding. This led to him being the prime suspect for much of the case.
Ketchup is indistinguishable (at least technically) to blood in Mitadake High.

Web Comics

Happens in a guest artist story on Sluggy Freelance, where Zoe thinks Riff and Torg are dead, but really they went a little crazy eating tacos and got hot sauce all over themselves.
In Weregeek, Joel does this when he finds 'blood' on the ground. Apparently it's peppermint.
A Dragon Tails strip had Bluey trying to re-purpose a machine that's just a series of giant blades into an automatic hat remover. He demonstrates it on a watermelon. The final panel has Bluey covered in red, commenting "Oh come on, you know this isn't as bad as it looks."

Web Original

Subverted in Thy Dungeonman 3. The "ketchup" in the sandwich shop is, in fact, fake fake blood. You die unpleasently when attempting to consume it.
Crapshots does this with what turns out to be jelly (of an unspecified but red flavour) in episode 16.
Inverted or... something in one Halloween episode of Weebl and Bob, where a talking jam is horrified because the substance is jam rather than blood.

Western Animation

Bugs Bunny loves this. He's done it to Elmer Fudd loads of times, and has done it Yosemite Sam at least once.
In one Tom and Jerry short that was particularly graphic even for that cartoon, Jerry makes Tom think he's been stabbed by dripping ketchup onto both a steak knife and Tom's chest.
Done in a Donald Duck short, where he and the nephews go camping. Donald puts ketchup on himself to get his nephews thinking he's severely injured, but gets bandaged up from head to toe, as is typical of those shorts.
- Sixty years later, they pull the same trick on him in a Mickey Mouse Works short.
Happy Tree Friends: "Flippin' Burgers". The immediate results are messy, and most definitely not just ketchup.
Parodied on Rocket Power, in the episode where the gang thinks Tito has been murdered:

Twister: Oh, man! There's blood all over that broken ketchup bottle!

Parodied on South Park, where "vampires" (kids which are pretending to be vampires because of Twilight) use "Clamato" (clam-flavored tomato juice) for their initiation ceremony. Butters believes it is blood and that he has really become a vampire. Believing he is unable to eat real food, he "suddenly" becomes insatiably hungry...
- Clamato is a real thing, by the way. (I don't know if it was the show's error or the troper's, but it's not clam-flavored, it's got actual clam broth in it.) It's popular in Canada.
An episode of The Magic School Bus has Ms. Frizzle, in a vampire phase, present the kids' parents with glasses of "blood". Keesha pointed out that it was obviously really tomato juice, but no-one listened to her.
Phineas and Ferb, of all shows, pulled this one off in "Hip Hip Parade".
A few Warner Brothers cartoons, such as The Heckling Hare and The Wise Quacking Duck, all demonstrate this trope.
Happens in episode 13 of Wakfu, which parodies several horror tropes. Sadlygrove is found amongst plenty red stains over the walls and floor, and the heroes think he's wounded or dead... except that he comments later, "It's a bad idea to eat red berries while seasick."
Subverted in Episode 11 of Scooby-Doo! Mystery Incorporated. Fred returns from visiting a Blood Bank with a huge red stain on his mouth and shirt. Shaggy and Scooby nervously ask him if its blood, and he replies no, its juice. The subversion comes in when he goes on to mention they have bags of the stuff hanging all over the place. Yes,this show had Fred drink blood, thinking it was juice. The blink-and-you'll-miss-it quality of this joke is what probably got it passed the censors.
- In the first ever episode of the original series "What a Night For a Knight" the gang think the knight murdered the scientist as they see what appears to be blood on the carpet, it turns out it was red ink.
In an episode of The Simpsons, Marge tries to get Homer out of bed. In the process she folds the mattress in half with Homer still in it, a red liquid leaks from the mattress and Marge screams. It turns out it was a juice box Homer had.
- After Marge sells a house to the Flanders family, she steps inside to find the entire family lying on the floor, covered in what is assumed to be blood. It turns out that they were just taking a nap after spending all day painting one of the rooms red.
In the Tex Avery cartoon "The Cat Who Hated People" one of the cat's flashbacks features the cat being adopted by a family with a huge bulldog, one the owner leaves the bulldogs beats the living hell of the cat until the owner hears the noise, the dog dumps some ketchup on himself and an axe making it look like the cat killed him.
Spike tried to invoke this trope with a toy mouse in My Little Pony: Friendship Is Magic.
Finn and Jake also try to invoke this (again, with ketchup) in Adventure Time when the Cute King won't leave them alone until he thinks he's killed them.

Finn: Oh no! My bloooooooood~!

Codename: Kids Next Door: "Op. S.A.F.E.T.Y." Numbuh 4 uses this to trick a safety robot into thinking it had accidentally hurt his little brother Joey, causing it to short circuit, self-destruct, and the ship they were on to crash.
- Immediately subverted afterwards. After Numbuh 4 mentions it was ketchup, he believes he also applied this to one of his own injuries. Numbuh 2 says it looks more like blood, and 4 passes out at the realization.

Real Life

This actually happened to Australian Idol winner Casey Donnovan. She, her brother and stepsister were involved in a car crash on the way home from the local takeaway.