The knowledge that communication with another brain through words and/or body language is hard. It's very lossy and almost always the source of error when I think someone has just said something absurd or incomprehensible. I may be ignorant of their train of thought, but that does not mean it's inherently random.
I use this constantly to quickly identify non-surface-level differing usages of terms, or to tell when I'm interpreting a phrase someone said differently than they mean me to. Latest concrete example: a couple hours ago, when I suggested that a D&D 4e melee was not well supported by the rules at all, and specifically that "what would the DM do?" summed up my objections. My roommate replied that he would do exactly what he always did, which didn't jive with what I (thought I) was saying, and I immediately knew we were interpreting "melee" in different ways.
Either that's availability bias, or it comes up very frequently, since the most recent event was mere hours ago. :)
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.