Bug Prediction at Google

Here at Google, we have thousands of engineers working on our code base every day. In fact, as previously noted, 50% of the Google code base changes every month. That's a lot of code and a lot of people. In order to ensure that our code base stays healthy, Google primarily employs unit testing and code review for all new check-ins. When a piece of code is ready for submission, not only should all the current tests pass, but new tests should also be written for any new functionality. Once the tests are green, the code reviewer swoops in to make sure that the code is doing what it is supposed to, and stamps the legendary "LGTM" (Looks Good To Me) on the submission, and the code can be checked in.

via Bug Prediction at Google | Google Engineering Tools