They fall under 4 categories 1. World level Amateurs who turned pro too late (Many Cubans and Russians fit this bill) 2. World level Amateurs who didn’t take the pro game seriously and/or didn’t have the style to be great professionals. (Frankie Gavin) 3. Professionals who passed the eye test early on and looked great but had hidden weaknesses that only showed up later. (David Price) 4. Professionals who passed the eye test but had poor promotional backing and only got their step up or challenge fight when they were past prime. You also have overrated amateurs who were never up to much but got hyped and then exposed. Audley Harrison springs to mind in this category.