It seems to me that justice is in the most peril when the prosecutor is absolutely certain that the perp is guilty, but doesn't have any good evidence (which is not an unusual occurrence). So now he is faced with two possiblities, let the guy go and watch him kill/rob/rape again, or make up some bullshit that sounds good to a jury and that his public defender won't question too harshly. Something that sounds scientific even though it was never actually proven effective. Hair analysis, signature analysis, ballistic fingerprinting, eyewitness testimony, etc... There are people on death row that were convicted solely on circumstantial and unreliable evidence, and not a lot of it. Of course all this is a lot easier to accomplish if you can use the jury's inherent biases in your favor. Like you're trying to convict a 6"4' black man with tattoos all over his body and permanent sneer. Doesn't really matter if he's guilty or not, the jury is going to want to lock him up regardless.