I understand it this way. NTF means, the manufacturer does not find any non-conformity in the finished product under normal test procedures. Yet, the customer or the end user has found a non-conformity. Obviously, when the product gets returned to the manufacturer with field complaint, he is not likely to discover anything wrong or No Trouble Found.
To avoid this situation, many tier 1 suppliers adopt MEOST (Multiple Environment Over Stress Testing) on their products during development phase (or even later, too). If the product contains any electronics, ESD precautions are also added.
This is now a requirement of IATF 16949 also.
