On the previous thread I mentioned the Mind-Projection fallacy and "the opposite of stupidity != intelligence" as being most frequently referenced, but on reflection, I think a reminder of the strictness of rationality makes the biggest difference in practice.
This passage from Technical Explanation sums it up: "But the deeper truth of Bayesianity is this: you cannot game the system. You cannot give a humble answer, nor a confident one. You must figure out exactly how much you anticipate the Sun rising tomorrow, and say that number. [...] You cannot do better except by guessing better and anticipating more precisely."
At this stage in my life, I find it easy to avoid to dogmaticism. False modesty, uncertainty, vagueness, and skepticism are all much more seductive. I work as a policy analyst in state government, and am frequently asked to provide forecasts. The dangers of narrow prediction intervals are well known, but I am tempted to be cautious and not focus my estimate. The field I work in is notoriously uncertain, but I can't do better just by being vague. Confidence and uncertainty have to be precisely balanced.
Followup to: The Most Important Thing You Learned
What's the most frequently useful thing you've learned on OB - not the most memorable or most valuable, but the thing you use most often? What influences your behavior, factors in more than one decision? Please give a concrete example if you can. This isn't limited to archetypally "mundane" activities: if your daily life involves difficult research or arguing with philosophers, go ahead and describe that too.