There were so many... I learned programming by myself with limited resources when I was a kid. Eventually I got a formal education and there were a lot of aha

There were so many... I learned programming by myself with limited resources when I was a kid. Eventually I got a formal education and there were a lot of aha moments! For instance: 1. Writing a Scheme interpreter in C 2. Abstract data types and encapsulation 3. Functional programming and recursive algorithms 4. Groking OOP and design patterns (this took me a long time) 5. Understanding how processes and scheduling is done in an OS More recently, not really a "aha" moment, but Git has been a game changer.