A man in a battered dressing gown sits on a log by a thatched village, frowning at a sleek black bird-shaped device glowing in the air before him.

Original artwork, not a still

Mostly Harmless  ·  1992

Douglas Adams

A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.

Behind the line

What it means

Our interpretation.

It is Murphy's Law with a better bedside manner: however carefully you protect a thing, someone will find a use for it you never imagined. Engineers, safety designers and software testers quote it as the polite version of 'users will break it', and the sting is that the failure lies in the designer's imagination, not the fool's.

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In context

Spoilers for Mostly HarmlessShow the scene

The sentence comes from Mostly Harmless (1992), the fifth book in Douglas Adams's Hitchhiker's Guide series, which the first-edition cover called the fifth book in an 'increasingly inaccurately named' trilogy. It was the last Hitchhiker's book Adams wrote and the last he published in his lifetime. In the story Arthur Dent plans a sightseeing trip with Fenchurch, loses her in a hyperspace jump, and drifts on through a Galaxy that has gone wrong again. The foolproof remark has since become the standard citation whenever idiot-proofing is discussed.

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