Kill Evil Mutants!
How good are your tests? Would they still pass if the tested code was changed much? If so, there's probably a problem with your code, your tests, or both!
Mutation Testing helps reveal these cases. It makes changed versions of your code (mutants) and runs your tests against the mutants to "kill" them. Survivors, aka "*evil* mutants", imply that there are flaws in your code or tests.
This talk will tell you how and why to use mutation testing, and how it works, including some examples and tools for popular languages.