We always expect developers to test their own code and be sure that it works as expected before they release it for anyone else to use. However we should be aware that developers won't think of all the edge cases that might break their code, so a good software process includes another group (QA) that tests the software separately, and hopefully also reviews the code as well.
Why do we do this? It's because you can't be sure that just because code passes your own tests that it is well written. You can only be sure that it passes the various tests you have designed, which is likely a subset of all possible tests it ought to pass.
Bruce Schneier wrote a piece that talks about this from the security perspective. If a system fails initial testing, we know there's a problem. However if it passes those tests, we still don't know it's secure. There still might be other vulnerabilities, perhaps even obvious ones. Only through exhaustive testing and review of the code, can we gain any sense of confidence in the security of a system. Even then we would still want to include some monitoring of the system as vulnerabilities might be discovered at a later time.
Each of us has some limited amount of knowledge about the technology we work with, but there is one common thing we all share: we can all learn more. Those limitations of our knowledge carry over to the code we write, and mean that we won't necessarily completely test our code to cover all possibilities. We might not even remember to test for some things as our work gets more complicated.
We need to work with others to ensure enough code coverage and have some level of confidence in the quality of our work. Today's systems are just too complex for one person to completely understand. Once you accept that and learn to work in a team, critiquing the code of others and accepting criticisms from them on your work, you will find the quality of your software will rise.
Steve Jones
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