"it's got to have a punchline, or at least a twist, to become a true and memorable UM."

Sez who?

What's the punchline or twist to "Car stolen with granny's corpse on the roofrack"? They left the car while they had coffee. Came back. Car + corpse gone. That's the tale. Some embellish it with difficulties about reporting death to authorities &c; but these are outward limbs & flourishes. No punchline or twist to main tale SFAICS. And these additions don't lead anyhow to an actual identifiable punchline in any version I can find.

But it's still probably the best known UM.

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