The problem seems to be that many jurors just do not have a proper conception and understanding of proof, and appeal courts show extreme deference to the judgement of jurors. So somebody can be convicted of murder based entirely on speculation rather than proof.
One sign is a trial of extreme length. That happens when there is no proof and it’s all guesswork what really happened, or worse it’s clear a person is innocent but they are still found guilty.
There is also evidence that jurors who are in favour of the death penalty are precisely those jurors who will convict when evidence of guilt is lacking, so the death penalty compounds the problem.
Trial by Media 