Psychology is such an interesting subject, and frankly, there are many concepts that I will remember five years from now. The most prominent phrase that has stuck in my head since first learning it is "correlation isn't always causation."
This information would have helped you in a case of sibling rivalry. I am sure many of you have experienced being blamed for something your brother or sister did. You could have explained that just because such and such was correlated, it doesn't imply causation.
I never noticed how often I used to connect two things together because they seemed at least remotely related, even though they were so far from the truth. Now whenever I notice two things that seem similar, I stop and think logically about what the true cause may be rather than assuming that A caused B. It is actually something that is very handy, because it is so easy to jump to conclusions too soon and make mistakes. We are cognitive misers after all. Knowing this information gives me the chance to determine the real cause and make better decisions. This is definitely something that will stay with me down the road, hopefully even longer than five years.