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.
Exploited: Bob has a crippling phobia of blood, and Alice deliberately squirts some ketchup on the table so that he'll think it's blood and pass out, because she needs him out of the way for some reason.
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!"
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.
Played For Laughs:
Bob is a full-time practical joker and one of his favorite trick is using ketchup as fake blood.