When something malfunctions by virtue of not existing at all. Taken (but not precisely) from "Hitchhiker's Guide to the Galaxy". In the original usage, a spaceship existed for a while and then spontaneously disappeared.
Bob: Can I borrow your car?
Bill: Nope. Sorry. It's having a total existence failure.
A statement used to counter "I know what you are, but what am I?" if you wish to continue verbally bitch-slapping them. Also has a 30% chance of making them shut up in confusion of trying to determine the meaning.
1. Idiot (After verbal bitch-slapping): "I know what you are but what am I?!"
You: "I am a figment of your imagination, therefore you are non-existent."