If you write software and are serious about keeping it secure, you should be fuzzing your code. Fuzzing is a form of testing which helps you find hidden vulnerabilities before the bad guys do. The easiest way to get started is with http://fuzzbuzz.io . I highly recommend it.
I found this technique to be extremely helpful in finding non security related issues as well. Together with a similar technique called "random testing" where you write two implementation of the same algorithm (one simpler / inefficient), fuzz both, compare results.
