Bogus scientific evidence is in there somewhere too, but I agree - your point is more fundamental. People get a hunch - I just know X did it - and they stay with it. Maybe it was useful on the African plains several hundred thousand years ago and maybe it is useful in most places now but it's not great in detecting criminals. Still, never mind. We have the trial process to keep everything tickety boo and that works just fine.
What is amazing is the comment from some, "What? You expect me to believe in a wide ranging conspiracy?"
That is a bit of a canard, though. It actually doesn't take a wide ranging conspiracy... and the fact that there are wrongful convictions from investigation, to prosecution, to conviction to long-term incarceration should put to rest those sorts of arguments.
But back to point.
It bears repeating - if police are always right (my experience is that police are mostly right!), there would be no need for investigations, trials or appeals.
The case which bears repeating is the Susan Nelles case in Canada, a nurse accused to murdering children in a cardiac unit of an Infants Hospital.
http://en.wikipedia.org/wiki/Susan_Nelles
After Susan Nelles was arrested, the deaths stopped. (It never crossed the minds of investigators that an unknown perp may have stopped because of Nelles' arrest!)
However, it was the reason for Nelles' arrest which was at interest in the eventual Royal Commission....
Nelles asked for legal counsel when she was arrested. Her request was interpreted by the investigating police officers to be an indication of her guilt, but the court later ruled that such requests should not be interpreted as evidence of guilt.
The story went that the arresting cop knocked on her door and asked if he could come in. She said "yes", meaning he then did not need a warrant. Over coffee she began to get the idea that he suspected her and she asked for a lawyer. He arrested her.
Can you imagine what would have happened to either of Knox, Sollecito, or Lumumba if they'd insisted on a lawyer? All three had been threatened with violence, and in Lumumba's case one paper reports him saying he'd been beaten. Knox had been slapped simply for not remembering what the cops needed her to remember.