Second, cops and prosecutors. Cops will sometimes lie on the stand. Low-grade, poorly trained departments are more susceptible than others. It's usually some drug dealer who they've watched for years and know he's guilty, but never quite catch, so they help the evidence a little. Thing is, you may eventually get someone who isn't guilty.
Prosecutors get zealous.
The reasons I think are, at least sometimes, psychological. They're trying to put a guy away for life and, unless they're completely conscienceless, they have to think he's guilty, not just probably guilty. So they persuade themselves. Third, defense attorneys. They figure their job is to get the client off.