How does a large company make rookie mistakes which leave security holes? [closed]

Question

Sony was recently hacked with a SQL injection and the passwords of their user's was stored in plain text. These are rookie mistakes. In such a large company, how does this pass QA? How do they not have better teams than to know better than this?

The sheer size of the company that was hacked makes this different. It affects all of us because we all may one day find ourselves on a team that is responsible for something like this, and then we get the ax. So what are the factors that lead to this, and how do we prevent them?