As a historian, I have often found that the worst problems come not from the things that we don't know, but from the things everybody knows, that are dead wrong. Errors that have been repeated so many times they have attained a patina of authority which can be almost impossible to dislodge with facts.

And it's not just the "common herd" that fall prey to these -- there have been noted cases in which one erroneous statement has been cited by so many scholars, who were subsequently cited by other scholars, that subsequently uprooting the error became a major undertaking, and even reputable scholars would resist tooth and toenail.

And don't even get me started about the problem of commonly-held fallacies and fiction writing. You simply can't fight what Everybody Knows in fiction -- it's either bow to it or sidestep it.