This is a question that should be asked whenever there is a production defect. Not for playing a blame game on a QA person. But to identify what went wrong in our software development and QA process.
In my previous job this is a point where we used to do root cause analysis and to do CAPA - corrective and preventive actions.
Also in the recent Agile and in Shift Left Test Strategy, I will strongly say that one cannot simply ask only the QA this question. This should be asked to a developer of that feature and also to the Lead developer who did the code review for this feature.
Also when doing the root cause analysis you cannot simply stop at the very first answer. You should continuously ask questions based on the answers.
How this defect got missed in QA?
-We have not covered this scenario in our test case.
-Also developer has not written proper Unit/Integration tests to cover this edge case.
Why this scenario has not been covered in the test case? and also Why this scenario has been missed out during test case review?
Why developer has not written any unit/integration tests for this edge case?
-We have not got enough time to cover more scenarios or to do proper test case review or to write unit/integration test case.
Why we don't have much time to do this activity?
In Sprint planning, we have taken lot of cases/feature from the backlog for this sprint which actually created some time constraints for both dev and QA.
We have taken features from the backlog based on our story points for each feature/story. So, Why we have not properly assigned story points for those features?
-Yes we should be more careful when assigning story points for the features/stories that we are gonna develop for every sprint.
So the final answer is the CAPA that we have to take a look at very carefully and it should be implemented for every sprint going forward.
Disclaimer: The answer is applicable only for people and organization who have got some broad mind to accept their own mistakes and willing to correct them as early as possible.
This is not applicable for some companies/people who are more interested to play a political blame game or where they want to treat QA as Scapegoats for the production defects.