I think anyone who thinks that the police don't lie occasionally when on the stand, or that they don't often exaggerate the truth to fit the prosecutions case, are naive.  But who would have thought that it happened so often that the police actually gave a name to it:

But a closer look at those prosecutions reveals something that has not been trumpeted: more than 20 cases in which judges found police officers' testimony to be unreliable, inconsistent, twisting the truth, or just plain false. The judges' language was often withering: "patently incredible," "riddled with exaggerations," "unworthy of belief."

The outrage usually stopped there. With few exceptions, judges did not ask prosecutors to determine whether the officers had broken the law, and prosecutors did not notify police authorities about the judges' findings. The Police Department said it did not monitor the rulings and was aware of only one of them; after it learned about the cases recently from a reporter, a spokesman said the department would decide whether further review was needed.

And of course, this is just the cases that were found.  How often to judges take police at their word?  This snippet I found to be incredibly telling:

In one case, the officer explained that he had a special technique for detecting who was hiding a gun. He had learned it from a newspaper article that described certain clues to watch for: a hand brushing a pocket, a lopsided gait, a jacket or sweater that seems mismatched or out of season.

That was one reason, he told a judge, that he was certain the man he saw outside a Brooklyn housing project last September was concealing a gun. The man, Anthony McCrae, had moved his hand along the front of his waistband, as if moving a weapon, the officer said. Sure enough, a search turned up a gun.

The judge, John Gleeson of Brooklyn federal court, asked the officer, Kaz Daughtry, how successful his method had been in other cases.

Officer Daughtry replied that over a three-day period, he and his partner had stopped 30 to 50 people. One had a gun.

One person had a gun out of 50?  What is especially troubling about this is that it would appear on the face of it that he was doing a good job.  After all, in this sort of situation, if you don't question his technique, then you only hear of the successful times he found a gun.  But the reality here is that he hardly ever found a gun, which means that he was searching people without reasonable cause... which is a clear 4th Amendment violation.  He invented procedure from a dubious source... some random newspaper article.  Was this technique approved by his supervisor?  Not only that, but it is revealed later in the article that he was behind the suspect, making the entire story highly unlikely.

All of this seems even worse since there is apparently no accountability.  Prosecutors don't pursue any charges for purgery since they rely on the police for most of their cases, and police supervisors are either never informed of these problems, or choose not to pursue investigations if they are informed.  Should these cops be fired?  Perhaps not.  But at the very least when these incidents occur, they should be talked to, and perhaps take a refresher on acceptable policy for searches.  Of course the reality is that they're turning a blind eye because they want to.