in the context of a best practice on how to perform a no blame post mortem review, with a view to process improvement. 
If a major issue is found in an application after it has been released (a test escape) who/what should ultimately be considered responsible?

 - The developer for introducing the issue?
 - The tester for not finding the issue?
 - Both the developer and the tester?
 - No-one (it's the system that failed, not the people)
 - Everyone

Thoughts ?

Note: No actual persons were harmed in the writing of this quesiton