In Series 3, episode 1, DCI Grey says that Ken Barnaby (the father) sending his hand through a blender, that will introduce reasonable doubt to a jury. How? If I'm on a jury and the prosecution says that the cops called the father asking him for fingerprints and the same night he shredded his hand, I would not think that was the action of a man who thought himself innocent.
It certainly seems like evidence of a guilty mind, but that in itself is not proof of anything. It's circumstantial at best. For all the jury knows, it could have been exactly the kind of stupid accident Ken claimed it was, the likes of which people hear about every day.
The point was that Barnaby's fingerprints were the only evidence linking him to the killing. Even if it is suspicious as hell, he still couldn't be convicted (unless they found another piece of evidence). Barnaby could flat-out say: "I shredded my arm because I distrust the police and don't want them to have my fingerprints on file", and he would still walk.
In Series 1, episode 6, why does Mark change his mind about thinking Luther killed Zoe? It doesn't seem like we're given any reason for him to do so. Alice drags Mark at knifepoint to where Luther is hiding out, Mark sees Luther and starts punching at him, and then suddenly Mark is sneaking into Reed's locker because Luther and Zoe want him to. At first I thought maybe he figured if the diamonds are in the locker, that would somewhat corroborate Luther's story, therefore he didn't really believe him until he found the diamonds. But considering the fact Luther told him the locker combination, Luther could have put the diamonds in the locker. So basically he just randomly changes his mind for no reason.
Also, a bit of a minor one, but why was Reed so completely unsuspecting about catching some random civilian wandering out of the locker room? He was clearly concerned someone would steal the diamonds, based on the fact he put a camera in there, yet he was completely convinced by Mark's excuse about looking for the bathroom. I guess, to be fair, going into a locker room to look for the bathroom is fairly reasonable behavior.