Vagueness People see vague information as uncannily descriptive. Everyone thinks they do more of the work. Bread always falls jelly side down. Base rate departures, e.g. cancer remission. Psychological experiments find that people tend to put too much trust in their own opinions.
Overconfidence encourages confirmation bias.
Overconfidence can also encourage insufficient anchor adjustment, i.e. getting stuck with an initial biased estimate. Overconfidence also encourages hindsight bias, believing after the fact that you were right all along. Don't just use the most easily available information, e.g. Web. Beware of other people's overconfidence. Tendency to misremember your earlier attitudes based on later knowledge of outcomes.