Once you accept that “stuff” happens in even the best of machine shops, you can begin to assess where the problems are likely to occur and start the long difficult process of fully understanding the capability of your technology and your people. The difference between conformance and non-conformance starts with the tolerances, finishes, and geometric form control shown on the drawings.
If you call for a tolerance of plus nothing and minus 2 tenths on a part, you better know that your machines can hold it. That ace operator you have total confidence in actually gets vacation days; machines require repairs. Work flow might require an operation to be done out-of-sequence or on an alternate machine.
You also need to understand the “why” of your requirement. Take fit diameters for bearings, for example. The bearing supplier has recommended fits for different types of service; what is acceptable for one application is unacceptable for another. If you select the values to put on the drawing, you should be able to “defend” them when someone asks for dispensation on the “minor” deviation the inspectors report.
The evaluation process is an excellent time to ferret out root causes. You cannot solve problems if you do not find out the real reasons behind the defect. We once chased a non-cleanup during tooth grinding right into a full root grind and eventually scrapped the part before discovering the root cause was not heat treat distortion, but sloppy hobbing before heat treat. Fixing that problem eliminated the clean-up issue and cut grinding time by a factor of five.
No one likes admitting mistakes, yet the job security of the entire team depends on determining root cause and making sure that a similar situation does not reoccur. And if you cannot prevent reoccurrence, at least you can anticipate the problem and make allowances in the overall process plan.