In the annals of wrongful convictions, there is nothing that comes close in size to the epic drug-lab scandal that is entering its dramatic final act in Massachusetts. About 23,000 people convicted of low-level drug crimes are expected to have their cases wiped away next month en masse, the result of a five-year court fight over the work of a rogue chemist. The prosecutors didn't want the scandal to end like this. They fought for a way to preserve the convictions. The chemist, Annie Dookhan ... worked at the William A. Hinton State Laboratory Institute in Boston for nearly a decade before her misconduct was exposed in 2012. She admitted to tampering with evidence, forging test results and lying about it. She served three years in prison. [It] is not entirely clear why Dookhan ... felt compelled to change test results on such a massive scale. She was by far the lab's most prolific analyst, a record that impressed her supervisors but also worried her co-workers - a red flag that went overlooked for years. She also maintained friendly relationships with prosecutors, even though her role was to remain objective. Lab scandals have undermined thousands of convictions in eight states in the past decade. Critics say forensic chemists feel a duty to help prosecutors rather than remain neutral. Because of the system's reliance on plea bargains to keep cases moving, defendants often don't have a chance to challenge results from drug labs.