Try programming a piece of software with zero holes in it. It's not easy, even in a small release, to cover all the holes without testing. However, internal testing is never enough, as most of those involved in the test process will already be familiar with the system and its idiosyncrasies. Most of the bugs will be found by novice users using the system for the first time. Even expert users aren't as useful as testers, since they often find problems then try to work around them themselves. This has always been Microsoft's testing strategy, and while they certainly shipped Vista with more than its fair share of bugs, every version has had them, and each new version will too.