Zero doubt. Ya know, guys like Bernardo and Pickton.
There is no such thing tho. We (as in you and me) believe pickton is 100 percent guilty because of what the police told us. But - what if they lied? They've done that before when they wanted to catch someone they "knew" was the guilty guy - fabricated evidence that made it look like someone was guilty when they weren't. And more than a few times i might point out.
Then there's cases like the guy who PRETENDED to be an expert on child injuries (in bc i believe) and put dozens and dozens behind bars arguing that in his professional opinion he was 100 percent sure the evidence pointed to child shaking etc and it was all 100 percent bullshit - but the jury believed him (and why not, he's the "expert") and locked the people up. It was years later when he was exposed.
We can never be 100 percent sure and it's VERY easy to manipulate us. WE have to rely on the word of other people. So we should always assume they MIGHT be lying and make sure that if it does turn out they are some day that we havne't killed an innocent man.
Fun fact, they are now pretty positive the last man killed by capital punishment in Canada was innocent.