Tiger teams
People hired to demonstrate vulnerabilities in systems by exploiting those vulnerabilities.
Complexity: These people are usually honest, but sometimes they are not. In addition, they often fail to properly repair the systems they try to break into, thus leaving residual vulnerabilities. Their skills vary widely, from rank amateurs using off-the-shelf software - to true experts with a high degree of sophistication. It is often hard to tell which is which unless you are an expert.