This story was written by a staff writer for the Washington Post who ended up on jury duty:

Last week I was a juror in the trial of a man accused of selling a $10 bag of heroin to an undercover police officer. At the end of the two days of testimony, I concluded that the defendant was guilty beyond a reasonable doubt. I also concluded that he should be acquitted.

You can read the whole story here; the bottom line is that police fabricated evidence to make their case stronger, and the jury acquited.   The writer's key insight:

I believe they had the right guy, too. But the willingness to cheat, I think, is a poisonous corruption of a system designed to protect the innocent at the risk of occasionally letting the guilty walk free. It's a good system, fundamental to freedom. I think a police officer willing to cheat is more dangerous than a two-bit drug peddler.