The moment a person forms a theory, his imagination sees in every object only the traits which favor that theory.

Once you have an explanation, your attention starts working for it. Evidence that fits gets noticed and remembered, evidence that does not slides past unrecorded, and the theory feels better confirmed every week even though nothing has been tested. The dangerous part is that this happens with no dishonesty at all, which is why intelligent people are not protected from it.

When you feel confident about a diagnosis, whether of a person, a market or a fault, write down what you would expect to see if you were wrong. Then go looking specifically for that, since it is the one kind of evidence your attention will not gather on its own.