It's a controversial subject, but we live in a harsh world. Some police officers lie. Some have a documented history of misconduct, including lying or other acts that call their truthfulness into question. If they do, this history should be available to the defense so that the officer's credibility can be tested in court.
In 1963, the U.S. Supreme Court held in Brady v. Maryland that police and prosecutors are legally bound to turn over so-called "exculpatory" evidence to the defense before trial. Exculpatory evidence is most anything favorable to the defense. It can be evidence that the defendant is innocent, that someone else could be guilty, or that there were mitigating circumstances. Or, it can cast doubt on the reliability of prosecution witnesses.
When Wilbert Jones was 19, he was arrested. He was convicted of aggravated rape based entirely on a highly questionable identification by the victim, according to the judge who recently released him. The woman, a nurse, had picked him out of a lineup months after the crime and was careful to tell police that her assailant had been taller and had a "much rougher" voice.