first of all, you have to do your own quality well. only in this way can you have confidence. In fact, there are many examples of deliberately finding fault. Sometimes, flies will be deliberately put in the dinner bowl to find fault. At this time, you can change a bowl for him, but you must tell him that it can be changed, but the quality is absolutely no problem. If he wants to do something else, such as sue you in the Quality Management Bureau and blackmail you, you can call the police to solve the problem. But only if you pass the quality test can you be confident.