As Donald Rumsfeld said, there are the Known Knowns, the Known Unknowns, and the Unknown Unknowns. He considered the last category to be the most dangerous.
I htink the most dangeous category is the Unknown Misknowns - the things we know are true and valid, and that we are completely wrong about.
As an example, here are some well know facts that everybody knows are true
- Napoleon was short
- Vikings had horns on their helmets
- Danish Pastries come from Denmark
- Chameleons change colour to match their surroundings
- Mussolini made the Italian Trains run on time
- When Olde English was spoken, people said "Ye", not "The"
- Bats are blind
- Sugar causes hyperactivity in children
- The tongue has Taste Zones
- Humans have 5 senses
- The declaration of independence was signed on the 4th July
Luckily, believing these are true is not dangerous - very little harm can come if you think bats are blind, or Vikings wore horned helmets
However once upon a time (17th century), everyone knew that tobacco was healthy, while the tomato was considered poisonous. Those unknown misknowns could kill you.
In any Knowledge Management system, there needs to be constant challenge to the Unknown Misknowns - the assumptions and misconceptions that "Everybody Knows"