Thermal faults come first: hot spots on a plate, a housing that gets uncomfortably warm at the grip, a thermal cutout that trips in normal use because the sensor sits in the wrong place. All three are geometry problems, and geometry is what a mould fixes permanently.

Mechanical faults follow: hinge feel that degrades after a few thousand cycles, a cord that fatigues at the strain relief, a swivel that develops play. These show up on a flex or cycle rig, not in a functional check.

Both categories cost very little to correct while the design is a prototype and a great deal once the mould exists in steel. That is the entire argument for spending roughly thirty days on validation before tooling rather than discovering the same faults through warranty returns.