Harvard thought it had a cheap copy of the Magna Carta. It turned out to be extremely rare.
Briefly

Harvard University, presumed to possess a low-value copy of the Magna Carta, has discovered its document is actually a rare version from 1300 issued by King Edward I. This version is highly significant, contributing to world constitutional history as it represents a cornerstone of legal principles. Medieval historians David Carpenter and Nicholas Vincent verified its authenticity using advanced imaging techniques, revealing its true importance and value, contrary to Harvard's previous assumption of its insignificance.
"First, I'd found one of the most rare documents and most significant documents in world constitutional history... it was astonishment that Harvard had been sitting on it all these years."
Read at Boston.com
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