OzPeter... at least your naieve colleague was willing to be honestly thorough in his testing. I was reviewing a design late last year. It consisted of a single set of togglable switches that could be passed to a business system to adjust its behavior. There were 32 switches. I said to the designer, "You realize that you are creating two to the 32nd settings?" Without blinking they said, "Yes, it is what we need." I replied, "Do you intend to test all those settings?" They looked disdainfully at me and said, "Yes, QA will do full regression testing."
I had similar conversations with management and developers. All of them gave the same sort of reply, and telling me not to be concerned. Despite my efforts, developers coded the logic, QA approved it, and it was implemented by authority of management.
I learned two things: (1) These folks can't do basic arithmetic; and (2) they have no clue what "full regression testing" means.
Perhaps the third thing I learned is that I am clearly not communicating with these folks! :-(