I think that's wht good programmers do - they test continously and at every opportunity. "Test early and often" is their motto. It's not that they have fewer bugs, but that the habit of continual testing has kept their bugs private, and consequently cheaper.
Hint: It's not from any XP book. Give up? OK, I'll tell you: "Software Testing Techniques" by Boris Beizer. What a fabulous book. I bought it because it discusses at length what the brunt of my talk at Linux World is on. He calls it structural testing (testing the code and design instead of the functional correctness). I highly recommend it. Did I mention it was written in 1983? Why do I keep unearthing these long lost tomes?! If anyone knows of any more, let me know!
I'll leave you with another quote from the book that I just loved:
There are some persons who claim that they can write bug-free programs. There's a saying among sailors on the Chesapeake River, who sandy, shifting bottom outdates charts before they're printed, "If you haven't run aground on the Chesapeake, you haven't sailed the Chesapeake much." So it is with programming and bugs: I have them, you have them, we all have them - and the point is to do what we can to prevent them and discover them but not feel guilty about them. Programmers! Cast out your guilt! Spend half of your time in joyous testing and debugging! Thrill to the excitement of the chase! Stalk bugs with care, and with method, and with reason. Build traps for them. Be more artful than those devious bugs and taste the joys of guiltless programming!
Oh, did I mention that these lovely quotes were from the introduction? It gets better! Sure, some of the book is outdated, but there are several great nuggets of information to be had. I'm going to go run off and do some guiltless programming right now.