one of the best cases: it turns out not to be a supplier´s issue, but a process issue. For example, a machine has been recently repaired, and changes in the machine settings have modified the process, in such a way that the components that previously were normally assembled are no longer fitting properly in the machine fixtures. Adjustment of the machine solved the issue.  

it is not your problem, but it is someone else´s problem: changes or random variations in parts produced by another supplier are affecting the process, but the parts produced by the company that we are representing have been identified as the probable cause.

it is your problem, caused by variation in a product delivered by one of your suppliers. A costly example: oxidation of stamped steel parts is reported by the inspection department at the industrial plant. Packaging of the product has been recently improved, by adding stretch plastic wrapping of cardboard boxes, which are placed on a pallet. The plastic wrapping keeps humidity inside the boxes, and the water-based lubricant and coolant used in the stamping presses is causing the oxidation. The coolant was identified as the root cause of the oxidation, and control of its composition solved the problem.

it´s in the print: a dimension in the mechanical design is taken from a base line that does not correspond with the functional surface that is actually important for the assembly process. A transfer of dimensions and tolerances is necessary to solve the issue and prevent further problems. The assembly fixture is temporarily modified, in order to use existing inventory.