The deeper issue with forensic science lies not in malfeasance or corruption—or utter incompetence—but in the gray area where Yezzo can be found. Her alleged personal problems are unusual: Only because of them did the details of her long career come to light. And yet the career itself is not as unusual as one might wish. It highlights how tenuous many forensic findings can be; how easy it is for prosecutors to make them appear solid to a jury; how closely some analysts work with law-enforcement colleagues, to the point of alignment; how rarely an analyst’s skills are called into question in court; and how seldom the performance of crime labs is subjected to any true oversight. All of this combines to create a dangerous prosecutorial weapon.

Brandon Garrett, a professor at Duke University’s law school and the author of Autopsy of a Crime Lab, told me recently that when he examined the forensic testimony in hundreds of wrongful convictions, he found “a blizzard of error.” To take a single metric: More than half of those exonerated by post-conviction DNA testing had been wrongly convicted based on flawed forensic evidence.

When asked to explain why forensics goes wrong, critics cite three factors. First, some commonly used forensic methods have not been rigorously evaluated; their validity has not been established. Second, the overwhelming majority of crime labs are not independent but tucked into police departments or state law-enforcement agencies. They depend on law enforcement for funding and operate under ever-present financial and psychological pressure to collaborate in securing convictions. Third, no one from the outside is rigorously checking the work done by forensic analysts, who may or may not have adequate scientific training. Many labs participate in voluntary accreditation programs, but, Garrett noted, accreditation largely focuses on having the right procedures spelled out on paper; proficiency tests given to lab analysts are extremely easy. Only in a few states are the regulations and controls that govern clinical labs and hospitals extended to crime labs as well. The quality of the work done in crime labs is almost never audited.