This is a great question. To me, the only answer is (as I believe you're alluding to) "who" is at fault is not relevant when compared to "what" is at fault. In

This is a great question. To me, the only answer is (as I believe you're alluding to) "who" is at fault is not relevant when compared to "what" is at fault. In this case, the problem is the lack of proper systems level integration testing, and neither Alice's nor Bob's code in isolation, and the "who" that ends up being at fault should be the management chain of command that allowed the state of things to allow such a scenario to occur. As management decides to want to prevent such embarrassing and costly blunders, they should resolve to invest more in the process and tooling that prevents such situations from being possible. Of course, it is also possible for management to shirk such responsibility and push that responsibility (without corresponding process ownership) onto the ICs. It's quite common in low performing organizations.