1. Identify the host coin
Record country, denomination, date, mint, design subtype, expected composition, expected weight, diameter, edge, and normal orientation. A correct error name cannot rescue a wrong host-coin identification.
2. Describe what the metal does
3. Map it to production
Planchet errors begin before striking. Die varieties repeat on coins from one die. Strike and collar errors occur under press pressure. Edge-lettering errors may occur in a separate operation. Anything after the final production event is damage or alteration.
4. Build a disproof list
Test impact, abrasion, heat, chemical attack, corrosion, plating failure, vise pressure, jewelry use, cutting, grinding, polishing, and machine contact. The best hypothesis is the one that explains all evidence while surviving alternatives.
5. Escalate based on stakes
A low-value curiosity may only need a labeled holder and photographs. A potentially valuable wrong-planchet, mule, major brockage, famous variety, or transitional composition claim deserves specialist review or major-service certification.