This is such a great topic, I also like thinking about time and timing, etc... the 4th dimension, right? Most examples of hindsight involve little tiny itty bitty weensy decisions you barely even notice, which you repeat day in and day out, but those decisions change your life in some miniscule way or another when they add up -- like: "Gee, I'm glad I didn't eat that donut for the 500th day in a row, because now I still fit in my jeans."
Only rarely is hindsight a big, momentous thing that is marked by angels singing and gongs going off. Still, that does happen. I remember once helping a friend move, and that decision drastically changed my life's trajectory in a way I could never have ever predicted. I also remember once someone told me something I was very good at was "useless", and that made me decide to get good at something else. Totally changed my life.
I wish I had paid more attention in certain classes, also. Who you listen to and who you choose to ignore can, in hindsight, make a huge difference.
Being nice to people routinely is also a very good idea, because you never know when you are going to walk into a job interview and suddenly run into that person from 5th grade you treated badly. It happens. Routinely treating people badly WILL bite you in the butt one day. It just will.
I was just reading about Rosa Parks, and her decision not to stand up on a bus one day had a profound impact on the world that I'm certain surprised her. Wonder if she had made a different decision that day, what would have happened? It's all very It's a Wonderful Life, isn't it?