while its clear that your testing team should have their own 5 Ys as well, I think its reasonable for the manager of your team asking you the question: how do we prevent this in the future? The unfortunate reality of large companies is that sometimes the quality and behaviour of other teams is (to some extent/on some time horizon) out of your control, and so the question for any given team lead is often "what can I do differently". It does seem that in this case there probably was some mechanism that your team could have had its own internal testing process that needn't replicate the full responsibilities of the testing team but could at least have caught this issue.