The biggest one has to be design patterns in OO design! I was fortunate to have a great teacher which taught OOP by focusing on design patterns, dynamic dispat

The biggest one has to be design patterns in OO design! I was fortunate to have a great teacher which taught OOP by focusing on design patterns, dynamic dispatch, composition over inheritance, etc, etc, the really foundational concepts. I was too young and naive to fully understand all the concepts, I just thought writing Java was cool! Ah. Many years later, I really realized how to apply design patterns to solve real problems in a large production codebase and it all clicked then and in retrospect that teacher was really amazing, I just was too naive to realize it back then...