I agree that breaking the build is normally a very bad idea. I've gone after friends with Nerf weapons for doing this. :-)

But there are a few cases where it's a necessary evil. For example, when upgrading a large Rails application from Rails 2.3 to 3.0, you're likely to make hundreds of small changes before everything works again.

In this case, I create a new branch, prepend "BROKEN:" to each commit message, and record the number of unit tests that are currently passing. Once all the tests are fixed, I hand-test, add new tests for any regressions that weren't caught by the automatic tests, and merge back to the main development branch.