Correlate the reported bugs back to which requirements they relate to. Then check the test cases associated with those requirements to determine why these bugs were not detected during your formal test process.
There could be any one of a million reasons, but you won't find out why, and therefore what do do about it, unless you can cross-check and correlate your reported bugs/requirements/test cases/test results.
Once you've identified the common thread or cause, then obviously, you should fire him/her. [img]/images/graemlins/wink.gif[/img]
I came across a document the other day in my current project that would print up to page 15. When page 16 started to print I was blue screened. Not the common blue screen, but the one you get when the OS crashes totally. On investigation only one of our sites did this, but on all of the machines and with different printers. It was a GIF file that caused it. We cut our the pix and converted it to JPG then back to GIF and no more problem. I suspect an interaction with the drivers used at our site. But, needles to say, it was a show-stopper.