Developers, what was your 'Eureka ' moment in coding?

Learning to use a REPL. It's been at the core of my coding practice since. Especially using it as a debugger in an interpreted language was a game changer for me. It converts programming into a fast interactive game.

I don't know if I can narrow it down to one moment that was a game changer above all others.

Using an interactive debugger is definitely one of them, as is building and using reproducible local dev environments.