"What a silly question?" a CEO would say and call for a quality manager: -"Bring me that looser. I told him more than once that this is not acceptable - to produce the software with critical defects. It hurts our business and stay in way of expanding it beyond the limits of Solar System."
Many times I saw how managers point their fingers to a person who bears word "quality" on the working title. But I saw few of the latter who could really do something. What is it then? A person who to blame no matter if he or she could do anything? A sacrifice goat whose fate is all pre-defined?
And what is more important - what could we, quality professionals, do about it?
Once I've been at a meeting with a big boss who very convincingly explained to us that of all the corners of a famous triangle (quality-feature-time), quality is the most important. "We never compromise quality!" he told. I was in love with that idea and was full of esteem to that person by then. But... within several releases I realized that those were just words. He did not think so. Every new release features prevailed until application started to look like a monster, a biting monster, because users were not happy when it crashed in their hands way too often.
So, who that boss should blame for the quality?
Before claiming the answer to the question "who is guilty" let's focus on solving the problems that we have in development and management that led to defects be injected into the code and not found during testing. After that, have people directly responsible for letting those issues go analyze the way they work and provide you concrete improvement actions which will help avoiding similar omissions in the future. But, doing so, keep it impersonal. Let them know that it's ok to make mistakes until they learn from them.
As for the question, if you want to blame someone start from you. Just ask yourself what would you do about the problem? How could you affect the process, so the issue is not injected or removed within the production cycle? If you are not perfect, you will always find something to improve, to grow. Answers to above questions will provide you with some insights.
Another very important thing is not to start finger pointing until you know the root cause of a problem. Make sure you have argument to prove someone guilty. Otherwise you risk to severely under-mine the moral of a person in whose performance and contribution you are interested. There is nothing worse than thinking whatever you do - you'll get blamed. This is a killer to motivation.
So, the short answer to the question is to start from you. The more in-depth answer would be: find out the actual reason, have people involved analyze the problem and come up with action plans, make corrections to the process.