I have to take exception to the suggestion that planting evidence is impossible. The most famous recent murder case in Ky, the Trent Digioro murder, had it. The FBI expert witness lied under oath. Was caught, charged, admitted to it and was fired and sentenced.
The ky supreme court then overturned Shane Raglands murder conviction because the FBI lied.
That's not necessarily planting evidence, but you are correct that this kind of misconduct occurs more often than people want to acknowledge.
The FBI agent in that case was actually prosecuted for perjury, which is about as rare as anything I've ever seen in a criminal case.
Interestingly enough, that FBI agent's testimony was the first thing that popped into my mind when the FBI expert in Avery's case miraculously completed his testing of the vial of blood in time to offer his opinion at trial. It's a test that hasn't been utilized in a decade, and it was scrapped because it's not reliable, yet here we are at trial with an expert who is 100% certain that the defense's theory is wrong. Very suspicious.