For nearly a century, a foundational concept in chemistry has been quietly passed from textbook to textbook, taught to millions of students as settled truth — until researchers finally held it up to scrutiny and found it wanting. The error's longevity is less a story about one mistake than about the invisible architecture of institutional knowledge: how authority compounds itself, how the unquestioned becomes the unquestionable. This discovery invites us to consider not just what we teach, but how we decide something is worth teaching in the first place.