When Gmail's spam filters are working perfectly

When Gmail's spam filters are working perfectly, no one talks to us anti-spam engineers. But as soon as something goes wrong, our users, our friends, and even our Google colleagues who use Gmail for their corporate mail are sure to tell us. That's just the way we like it. Spam is not something people should grow numb to and accept as a fact of life. We *want* people to complain. That's the only way things get better.
http://googleblog.blogspot.com/2007/10/its-not-about-spam.html