A Bloody Mess/Playing With

Basic Trope: A mundane substance is mistaken for blood.
 * Straight: Alice walks into a room and sees Bob covered in blood, but it turns out to be ketchup.
 * Exaggerated: The room is absolutely soaked in blood, and it all turns out to be just ketchup.
 * Justified: Alice had heard news of a serial killer and was expecting the worst, and so her mind assumed blood as soon as she saw something red.
 * Inverted: Alice sees what appears to be ketchup smeared on the table, so she scoops some up with her finger and licks it. It's blood.
 * Subverted: Alice sighs with relief as she realizes that it has to be ketchup, except that it really is Bob's blood after all.
 * Double Subverted: ...No, wait, it's fake blood, Bob was just faking the scene to see Alice's reaction.
 * Parodied:
 * Nobody in the series believes ketchup, looking so much like blood, could really be made of tomatoes, and are thus terrified of it.
 * Bob is covered with ketchup all right... but he literally has ketchup for blood, and is an actual victim.
 * Deconstructed:
 * Bob was covering himself in ketchup because of its resemblance to blood, but Alice doesn't understand the implications of this and Bob turns out to have villainous sadistic tendencies as he turns on Alice and his friends.
 * Alice has PTSD, and assumes red liquids are blood.
 * Alice calls 911 after seeing the ketchup, assuming Bob is dead.
 * Reconstructed: Alice was about to call the police until she decides to check his pulse, which was okay, then checked the blood, realizing it's ketchup.
 * Zig Zagged: Medics arrive to a restaurant where carnage took off during a tomato-themed dinner, check every person if he\she's bleeding or not and have a wrong initial guess every time.
 * Averted: Alice sees ketchup everywhere, sighs, and goes to clean it up.
 * Enforced:
 * "We can't have real blood in this show! Let's have it be actually ketchup!"
 * "If we leave a drop of ketchup on the table for the season finale, the watchers will never know the difference!"
 * Lampshaded: "This ketchup looks an awful lot like blood!"
 * Invoked: Alice wants to scare Bob, so she covers herself in ketchup and runs up to him.
 * Defied: Bob makes a mess with the ketchup, but quickly cleans it up because he doesn't want Alice to mistake the scene for something more sinister.
 * Discussed: "That's... not actually blood again, it's ketchup, isn't it? Why are we always making these mistakes; it doesn't look that much like blood."
 * Conversed: "Oh come on, Alice, that "blood" doesn't even look like anything but ketchup!"
 * Played for Laughs: Bob is a full-time practical joker and one of his favorite trick is using ketchup as fake blood.
 * Played For Drama:
 * Alice's failure to discern that the substance is blood highlights her inability to identify details in general, a crippling fault that causes her much trouble in the story.
 * Alice, not wanting to live without Bob, is too grief-stricken to smell the ketchup. She shoots herself. Bob, hearing the gunshot, jerks upright...

Back to A Bloody Mess, and make sure to sniff it before you decide what it is.