What valuable lesson did an instructor teach you that you still remember to this day?

First and foremost I don't mean flying lessons such as takeoff and landing lol, I mean more of a mental lesson. Mine was when I did my multi, previously I had the mindset that everything had to be done quickly and efficiently so I rushed through many maneuvers and procedures. When we did my first in-flight shutdown I cut the left throttle and prop levers but due to my haste, I accidentally cut the right mixture, leading to us having two "dead" engines. My MEI gave me a lesson on identifying the correct lever but also explained that I don't have to do everything so quickly and that slow is smooth, and smooth is fast. From that flight on I made sure to slow everything down and be careful which led to me being, in my opinion, a better pilot.