If you're a Ruby developer you should check out Heckle: http://glu.ttono.us/articles/2006/12/19/tormenting-your-tests-with-heckle http://glu.ttono.us/articles/

If you're a Ruby developer you should check out Heckle: http://glu.ttono.us/articles/2006/12/19/tormenting-your-tests-with-heckle http://glu.ttono.us/articles/2006/12/19/tormenting-your-test... Heckle is a mutation testing framework for Ruby. I don't use it all the time, but my tests are definitely better now that I've tried it and seen what some of the edge cases are...