We learn many lessons from many sources in life. We learn from people, from books, from observations. We learn in schools from teachers and from our peers.
Rarely are our sources of learning permanent. People come and go. You graduate from schools (or move away from them). Although you will learn so much from each of those sources, there will be a final lesson from them, and that will be of the most powerful lessons.
When you are learning, especially in formal education, every lesson leads into the next lesson. Every new knowledge works with your existing knowledge. The most recent lesson you've learned is the freshest in your mind, but when the next lesson comes in, the previous one gets mixed in with all the other previous ones.
When you stop learning, though, that last lesson always stays there. It is the most recent knowledge and it stays on the top of your mind. There is nothing pushing it in with the rest of your knowledge (it sort of happens over time, but it still sticks out as an important one).
The last lesson I learned in my aikido practice was, "if you have to correct yourself in the middle of your technique, it is because your screwed up in the beginning." That is a vastly powerful lesson, not only for aikido or martial arts, but for most things in life. And it sticks out to me because it was my last lesson.
My last lesson in college was that being humble is only valuable to a point, but after that it becomes detrimental; you need to take pride in your work, believe that it is great, and show to other people how great it is. If you don't do that, you will drown in the sea of everybody else.
Last lessons are not the only lessons that stick out. There will always be particularly revelatory moments that are so powerful that they do not leave your memory. There will be some lessons that are so all-encompassing or so common that you are always using and always thinking about them. Last lessons are merely one kind of those powerful lessons.
What are your last lessons? What do you draw from them and how do they help you out?