A Rolling Stone article by Paul Solotaroff:

Inside the biggest law enforcement scandal in Massachusetts history: How the system covered up tens of thousands of falsified drug tests – and how two teams of crusading lawyers exposed the wrongdoing

This is a long detailed article. I am just hitting the highlights of how two chemists helped prosecutors railroad thousands of people for The War On Drugs:

Meet Sonja Farak:

Since 2004, when she started pilfering drugs from her longtime place of employment – the Amherst crime lab of the Massachusetts State Police – her addiction to stimulants has galloped away and grabbed the reins of her life. She's a chemist who performs forensic analysis of the street drugs cops bring in, running samples through complex machinery to determine the chemical makeup of each substance.

A drug addicted chemist who works in a drug lab. What could possibly go wrong?

Until recently, Farak has been a standout performer. In less than nine years, she's helped send away between 8,000 and 10,000 defendants. The only thing more prolific than her output is her drug use. Farak's been high since virtually the day she was hired.

Easy access to drugs in the crime lab is one thing. This is ridiculous:

The Amherst site was decrepit and woefully mismanaged. It performed no routine audits and placed no cameras in the halls; employees had carte blanche access to the drug safe. So rudderless was the lab that Farak smoked crack in the restroom and cooked batches beneath the site's one working fume hood. Legally unfit to drive home at night, she was nonetheless allowed to do sensitive tests on samples she'd smoked or snorted herself.

Meet chemist number two:

This is the second massive scandal in five months. In August 2012, a chemist named Annie Dookhan was busted for faking tens of thousands of drug tests at her Boston lab, always in favor of the prosecution.

No drug addiction here. Annie was simply a highly motivated criminal justice warrior:

Dookhan's crimes were long suspected by her fellow chemists. They bitterly denounced her to their supervisors: Not only was she sending the presumed innocent to jail, she was making all her colleagues look bad. A crime-lab chemist is deemed productive if he or she tests 100 samples a month. Dookhan was doing 300, and seemed to have never encountered a single sample that didn't test positive for drugs.

The total count for two chemists:

Between them, the two chemists had potentially helped wrongfully convict more than 32,000 defendants.

Prosecutors with too much discretion are the real problem:

In the past 40 years, DAs have been given excessive powers to bully defendants into plea deals. Add-on "aggravators" – charges of selling in a school zone or near a public park; carrying a gun or knife while dealing, etc. – have been drafted in every state to get tough on dealers and maximize the time they serve in prison. Ninety-five percent of all convictions in America are plea-bargained in lieu of trial.

This is not an isolated problem:

"This could just be the tip of the iceberg," says one expert. "Prosecutorial misconduct is rampant in America."

Dookhan's cases were settled first:

It took Segal three years to get the list of Dookhan's victims, and another year to finally win them justice. In January 2017, the Supreme Judicial Court delivered a landmark decision called Bridgeman II. It ordered the state to retry the falsely convicted or dismiss their cases in 90 days. That spring, every DA with relevant verdicts brought forth their lists of dismissals. In a single day, April 19th, 2017, almost 22,000 people had their Dookhan convictions waived – it was the most in U.S. history.

Next up Farak:

As in Bridgeman, district attorneys were given a brief window to identify and dispose of Farak cases. On November 30th, their window closed. That morning, Segal picked up a client named Nicole Westcott and drove her into Boston for the decision. Westcott, seven months pregnant with her second child, was an addict well-established in recovery.

The happy ending to a long nightmare:

When she got to Segal's office, there were reporters and lawyers packing a conference room. At 10 a.m., some of the numbers were in. More than 6,000 cases were slated for dismissal; three of those cases were Westcott's. Shaking with some amalgam of disbelief and joy, she stood and fielded questions from the press.

https://www.rollingstone.com/culture/news/did-falsified-drug-tests-lead-...

More examples of vindictive, out of control prosecutors from a recent Slate article:

The Innocence Deniers: When convictions are clearly wrong, these prosecutors don’t just hinder justice—they actively work against it

This OP is long enough. Here's one example of injustice from the Slate article:

In June 2016, the California Innocence Project filed a vindictive prosecution motion, arguing that the retrial of William Richards was “unconscionable.” A month earlier, the California Supreme Court had ruled unanimously that Richards’ conviction for the 1993 murder of his wife Pamela Richards should be overturned, as it had been based on false evidence.

Shortly thereafter, San Bernardino County District Attorney Michael Ramos announced his office would prosecute Richards for a fifth time. “There was no way that they had any legitimate purpose in prosecuting him,” says Jan Stiglitz, co-director of the California Innocence Project. “They would have to be deluded to believe he was still guilty.”

Ramos is a prosecutor with a national profile. A Southern California Republican, he is the board chairman of the National District Attorneys Association, a powerful group of more than 4,000 prosecutors that meets regularly with the Department of Justice to offer its views on criminal justice policy. The son of Mexican immigrants, he is the first Hispanic district attorney for San Bernardino County, a jurisdiction with a population of more than 2 million people.

https://slate.com/news-and-politics/2018/01/innocence-deniers-prosecutor...

At the end of the Slate story is a link to seventeen more cases of denied innocence:

https://slate.com/news-and-politics/2018/01/innocence-deniers-seventeen-...