Several forensic techniques presented in court for decades as identification evidence were never validated in the way the claim required. Bite mark comparison, hair microscopy and some pattern-matching disciplines developed inside laboratories serving prosecutions, and their error rates were unknown because nobody had measured them. When measurement was finally undertaken, the results ranged from modest to disastrous. What made this possible was not fabrication but an evidentiary standard that asked whether a technique was generally accepted in its field, a test that a field can pass by consensus among practitioners sharing the same untested assumptions. DNA profiling, developed in academic genetics with error rates estimated from the start, is the exception that shows what was missing.