I am quite convinced that far more damage is done by what people believe to be true than what they don't know.
The title of this week's Dividend Cafe comes from one of my favorite quotes, ever.
"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."
~ Mark Twain
(I attribute to Mark Twain because everyone does, but ironically given the substance of the quote, even this appears to be untrue, with most scholars believing there is no reputable source for who quoted these exact words).
The underlying message here is never more relevant than in investing, it seems to me. Plenty of people don't know certain things, and that has some impact at given times. But I am quite convinced that far more damage is done, not at the things that are that people don't know, but the things that people believe to be true (and act upon), when in fact they are not.