Testing is a critical activity; thorough and continual testing (including regression testing) leads to a stable product in which both the engineering team and the customer have deep confidence.
Here is an often-overlooked truth: complete code coverage testing is critical! Why? Simple—there are often hidden defects lurking in sections of code that never actually get tested (error handling being the typical example); the fact is though, they will be hit one fine day, and this can cause terrible failures.
Then again, unfortunately, testing can only reveal the presence of errors, not their absence; nevertheless, good and thorough testing is absolutely critical. Most testing performed (test cases written) tends to be positive test ...
