Who's fault is it?

Recently I met the situation when the bug was found by the end-user.Of course sometimes this can happen. In this case the programmers say that the application wasn't tested carefully enough. And testers say that they can't check the functionality without access to the source code and so on...
But in fact - who is to blame for that? Programmers or testers?
What do you think about it?

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To answer this I need to know so much more about the bug.
- Was it in an area included in your test coverage but missed by a test?
- Did you explicitly plan not to test that area?
- Was the product trialled, if so, how come it wasn't caught there?
- What severity was it? Showstoper, Major, Minor, Cosmetic?
- What circumstances does it happen under? Does it happen because of a conflict with a 3rd party app? Is it because of a certain type of hardware?
- Were you happy with the qualiy of the product when it was released? Was it released with known bugs?
- Would you have needed to spend a huge amount of time testing masses of combinations of factors to capture this bug?

There are many more questions I'm sure, but I think it's hugely important that we fully understand a defect, before looking at who might be to blame (if you work in that kind of culture) for it being missed.