Ask HN: What change in your programming technique has been most transformative?

As an example, the team I work on has been adding more precondition checking to all of our applications. The simple act of stepping back from the perceived data-flow and explicitly declaring what we believe should hold true has uncovered several bugs in our understanding of our applications.

We've likened it to having someone review a paper you've written: you often read what you think you wrote, not what's actually written.

This got me to questioning what others have found to be transformative in their development practices.