Although we've learned a lot in psychology this past semester, I think the one thing that will stick with me is something we learned early on in the course. That is, our common sense isn't always right. The rules of thumb our brains follow are necessary for us to function and process information, but may cause us to think something that isn't true. For example, naive realism may lead you to believe that the world is flat but science has proven that it isn't.
I'll remember this, and want to remember this, because it is applicable to my everyday life. Everyone gets things wrong sometimes. Not because they're unintelligent or inattentive, but because our common sense gets in the way. Because of this, I've learned to check and recheck facts and make sure I'm doing all I can to be as informed as possible. Just because I "know" something, doesn't mean I actually know it.