Family doctor Harold Shipman got away with murdering his patients for decades. He was one of the most prolific serial killers in history - but his hundreds of crimes largely went unnoticed despite a vast paper trail of death certificates he himself had signed.

Why do we sometimes fail to see awful things happening right under our noses? And how can the systems that maintain quality control in cookie factories be employed to prevent another doctor like Shipman killing with impunity?

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