The most frequently useful thing I have learned from OB is to update assumptions based on new information on an ongoing basis. I think this idea ties in nicely with that of standing against maturity, if maturity is taken to mean a certain rigidity, an inflexibility of purpose and outlook.
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.