Security issues can be identified using the stock-and-trade critical thinking skills of a tester.
Some time ago I had the pleasure of taking part in a security bug hunt for a new financial product. This was a product ready to go to market, a product that had passed all penetration tests and was now being handed to a crowd of external testers for a final attempt to 'hack' the product.
Against all their confidence I was able to 'hack' that product and use funds to which I should not have had access. However, once I reported the vulnerability, I wasn't believed and I was asked to repeat the 'hack' multiple times until the 'experts' believed I was achieving what I was reporting - they simply couldn't believe that their penetration test result was wrong.
Like many security talks I will tell you all about the tool I used to perform this 'hack'; Unlike many security talks this is not a tool you can install, rent or puchase - because it's my brain, but your brain is capable of doing the same.