Why testers can’t spot all the bugs
When a manager asks why a tester didn't catch a bug in production, the tester should first accept that it is their responsibility to look for bugs but not necessarily their fault if they don't catch them all. Bugs are usually well-hidden and even the developers who put them in may not have found them. Evaluation of the testing process is a way to give management reasonable explanation on why malfunctions still occur.
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