Generally after the tester writes the Test cases it is reviewed by the peer and then by the lead, sometimes we may also have a review by having walkthrough with the Architect and Business Analyst.
Secondly when we go for the execution phase, and when the tester logs a defect it will first be reviewed or a triage meeting we be done to validate if the defect is valid or not.
There is nothing called as a bad defect. If the defect is not valid( in terms if it is the actual requirement) then it is considered as invalid and not a bad defect.
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1) There are peer reviews, walkthroughs and inspections.
2) It is a high priority and/or severity bug. We attach a high priority or severity level in the defect report and our description often carries a sense of urgency.
3) Requirements Traceability Matrix (RTM) helps to ensure that all requirements are covered by test cases.

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Bad Defects:
In testing world there is no such word called bad defect. Bad Defect is a new terminology which we are using in now a days in testing. Here bad defect is used in two different scenarios. scenario.
Scenario 1: If a bug is not reproducible then it is called as bad defect.
Scenario 2: For the above scenario because it is not reproducble some might not fix it and further that might cause a big problem at production level. Now here if client finds that bug and send it across to the development site they will mark this bug as bad defect.
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