The biggest career lesson that I've had is that 80% of development is testing. Should you apologize for bugs? No. But if you're doing professional quality work

The biggest career lesson that I've had is that 80% of development is testing. Should you apologize for bugs? No. But if you're doing professional quality work, (as opposed to prototyping or researching,) a large portion of your time (as a developer) should be writing automated tests (like unit tests) to test the code you write.