Ten prototype units, hand-soldered and individually tuned by an engineer who knows the design intimately, can mask problems that a thousand-unit production run will expose immediately. Scaling from prototype to production isn't just "build more" — it changes the failure modes you need to design against.
Component tolerance stacking is the first place this bites. A prototype built from a single reel of components has tighter real-world tolerance clustering than a production run sourced across multiple date codes and possibly multiple approved manufacturers. A circuit that works reliably across ten prototype units built from the same batch can develop a tail of failures once component tolerances spread across their full datasheet range at volume.
Second is the disappearance of manual tuning. If a prototype relies on an engineer selectively hand-picking a resistor value or manually adjusting a trim pot during bring-up, that's not a production process — it's a bottleneck and a quality risk. Anything requiring manual adjustment at prototype stage needs to either be designed out (tighter tolerance component, active calibration in firmware) or turned into an automated calibration step at test.
Third is thermal behavior at density. A single prototype board sitting alone on a bench dissipates heat very differently than the same board mounted in its final enclosure, possibly stacked near other boards or in an enclosure with limited airflow. We always validate thermal performance in the actual production enclosure, not the open-air bench setup used during initial bring-up.
Fourth, and most overlooked: what happens when a component goes end-of-life or hits a supply constraint mid-production run. A prototype BOM chosen purely for bench availability can leave a production run stranded when a single-sourced part disappears. We flag single-source, non-recommended-for-new-designs, or allocation-constrained components during DFM review specifically so this gets solved before it becomes a production stoppage, not during one.
The common thread: scaling exposes every assumption a prototype quietly got away with. Catching them in a DFM review is a planning exercise. Catching them on a production line is a fire drill.