[HarHarVeryFunny]

You can test every line of code in your program, but how many people actually do?

It's one thing if you are just creating a throwaway prototype, or something so simple that you will naturally exercise 100% of the code when testing it, but when you start building anything non-trivial it's easy to have many code paths/flows that are rarely executed or tested. Maybe you wrote unit tests for all the obvious corner cases, but did you consider the code correctness when conditions A, then B, then C ... occurs?). Even 100% code coverage (every line of code tested) isn't going to help you there.

[simonw]

> You can test every line of code in your program, but how many people actually do?

In my mind, that's what separates genuinely excellent professional programmers from everybody else.

[HarHarVeryFunny]

I think it's perhaps more that you learn to write code that is easy to test and debug, consisting of some minimal set of simple orthogonal components, etc. You test every function of course, but learn to intuitively design out these combinatorial complexities that could nonetheless still be lurking, and pre-emptively include assertions to try to catch anything you may have overlooked.