I believe if a product has passed internal validation and verification but failed after being tested by an outside contractor, it's likely due to inconsistencies with testing techniques. Testing internally, the company may utilize somewhat different testing parameters, assumptions or possibly unintentional biases within their testing process whereas the outside contractor will most likely follow the strictest and standardized testing protocols. In general, I believe the first course of action would be to determine why the discrepancy occurred. This would entail comparing internal testing methodologies, inspecting testing equipment, and examining the results from both the internal testing and external testing. Due to the importance of patient safety and the necessity for regulatory compliance medical device manufacturers cannot simply ignore such discrepancies. Therefore, these companies may elect to hire an additional independent tester, replicate their experimentation or modify their product design as necessary.
This example highlights the need for all companies to establish consistent, defined validation practices. While internal testing is helpful, it is often insufficient on its own.

One dimension worth adding to this discussion is the role that supplier qualification and audit processes play in preventing this situation from arising in the first place. Ideally, before a contracted company even begins verification and validation, the hiring company should have already audited the contractor's facilities, equipment calibration records, and SOPs to ensure their testing environment is sufficiently equivalent, catching potential discrepancies before any formal testing begins rather than scrambling to resolve them afterward. When a mismatch does occur despite those precautions, I think it's also worth considering that the contracted company's failure might actually be surfacing a genuine product weakness that the internal V&V process was too familiar with to catch, essentially falling into a confirmation bias where internal testers unconsciously optimize conditions to match expected results. This is why having a contractor test your product is valuable beyond just regulatory checkboxes; an outside perspective with different equipment and assumptions is a real stress test. As noted earlier, walking through both sets of protocols side by side in a joint meeting is the most practical path forward, but I'd add that this meeting should include a line-by-line comparison of environmental conditions like temperature, humidity, and sample handling, since those variables are easy to overlook and frequently responsible for unexpected failures.