Best AH-HA moment when something suddenly made sense.

When I finally understood how the animations needed to be applied when importing a model with assimp.

My first rant!

There are too many... it started with assembler on a C64 and now with gradle and maven...

when I finally understood:

"Code to an interface"

and I still learn the deep deep implications of that on an almost daily basis

Regex.

The AH-HA moment was when I successfully wrote my first regex on a whiteboard.

It's small but when I discovered how to use for loops for the first time with arrays, never yelled out in surprise so loud in my life....

The biggest for me was Object Oriented Programming, in general.

I started out on a C64 (non-structured / procedural), and I pretty quickly understood low level operations for generating sprites and waveforms.

Then I eventually moved to Javascript and Haskell, Python and a little PHP.

The whole concept of classes with methods and variables just didn't make sense to me for a very long time, coming from a Functional/Procedural background.