Answer: split it in the contract, because after the fact both sides are arguing from memory.
- Define a defect — against the golden sample and stated tolerances, not "does not work".
- Define verification — who tests, with what instrument, at what resolution, and what record proves it.
- Define freight — who pays the return and the replacement shipment.
- Define remedy — rework, credit or replacement, and the timeline.
- Define the boundary — damage from installation or field use is not a manufacturing defect.