In Paul Fussell's Boys' Crusade, which I'm still slogging through like a good soldier (when I can bear it), there's an account of a typo that turned out to be a test of bona fides when German spies tried to pass as Americans:
"Every American officer carried, in addition to the metal identity tags around his neck, a laminated card with his photo. These cards had one curious feature: an uncorrected typographical error. The top of the card read NOT A PASS. FOR INDENTIFICATION ONLY. Someone preparing the disguises of the Skorzeny spies couldn't resist—some will say 'in a German manner'—pedantically correcting the spelling on the false cards issued to those masquerading as American officers."
An error that becomes proof of authenticity: It calls to mind the value placed on a knitting mistake as evidence of the genuineness of a sweater advertised as handmade. Or the mole that highlights a beauty's otherwise flawlessness. Or, more in keeping with Fussell's context, the scars that allow identification of a mangled body.