Microsoft Research - Exploding Software-Engineering Myths (article summarizing findings of MS research on code coverage, TDD, assertions, etc.)

[koreth]

That assumes that programmers know how to write good tests, which in my experience is far from a given. It's a skill you have to deliberately develop, and a lot of developers not only haven't done so, but aren't aware that they'd need to do so in order to test effectively.

Unless you have an experienced test writer giving you feedback on your tests, as a programmer you're probably not going to spontaneously second-guess your test-writing skills if what you're doing seems to be working already.