This is such a very important component of the whole equation (and one which, needless to say, is so often ignored within pro-guilt "arguments").
As an absolutely classic illustration of the point, one might consider the infamous "Phantom of Heilbronn" case in Germany, where police were convinced they had identified the DNA of a serial killer. The DNA of the same woman kept turning up at different crime scenes (including several murders). Mysteriously, the DNA of different (but unknown) male "accomplices" also turned up at each different crime scene. The police believed they were looking for an evil female serial criminal/killer, who had strangely employed the help of a different male accomplice on each crime.
It turned out that the woman's DNA hadn't come from the actual crime scenes at all. It was present on the cotton swabs that the police were using to gather evidence at the crime scenes. And it had been deposited there by the woman who packed the cotton swabs, by hand, at the factory where they were made.
Long before this revelation finally came to light, the police should have looked at the context, and asked themselves: what is the likelihood of there being a female serial criminal/killer working over a wide geographic area of Germany, who has a different male accomplice for each and every crime she commits? It should have been easy for them to, at the very least, question their conclusion in the light of that context, and looked for another, more rational explanation. But they forgot about context for a long time, and tied themselves into their bizarre theory (with embarrassing consequences in the long run).
http://content.time.com/time/world/article/0,8599,1888126,00.html
In the Kercher case, the police/PM should have been asking themselves, very early on, what the likelihood was of Sollecito's kitchen knife having been involved in the murder of MezzicleBezzicle? There was the bloody imprint of a completely different knife on a bed sheet in the murder room, and all of the wounds were fully compatible with the knife which had left that imprint.
In addition, there was little or no rational reason why that knife would ever have become involved, even if Knox and Sollecito had participated. There were plenty of "suitable" knives in the cottage itself, including a brand new set of good, razor sharp knives in their original box in Knox's room in the cottage. And it would have been a spectacularly bad idea of Knox and Sollecito to bring and use a knife which, had it really been used in the murder, and had subsequently been found to have been used in the murder, would so obviously directly implicate Sollecito - and by extension, Knox (in a way that, for example, the use of a knife from the kitchen drawer in the cottage would not).
On top of all that, there is also - as you point out above - a very, very serious problem with the chain of custody of this knife, and
all manner of problems with the handling, testing and analysis of this knife by the incompetent Stefanoni. All of this, put together, should have led the police and PM to look at the knife in its proper context, and should have led them to realise that, all things considered, the knife did not constitute probative evidence. But they had tunnel vision and confirmation bias, of course, so they were all too willing to tie the knife into their narrative involving "volpe cattiva" Knox and pliable Sollecito - up to and including the risibly stupid idea that multiple knives were simultaneously held to MezzaSchmezzer's throat and near-simultaneously plunged into her throat (no need to concern ourselves with the very obvious risk of the assailants injuring each other with each other's knives in such close proximity with a moving victim, of course.....).
Ultimately, of course, it was the role of the courts to assess all of this properly, in the proper context. It's now a matter of public record and Supreme Court reasoning that the Massei and Nencini courts manifestly failed to consider the knife evidence properly, within the rules laid down in the law and the code of criminal procedure. Had those courts properly assessed the knife in the proper context, they should have basically thrown it out as worthless evidence. Fortunately for justice, the Supreme Court (belatedly) righted this wrong and threw the knife out - along with every single other item of physical evidence, and every single piece of prosecution testimony - as fundamentally unreliable and devoid of credibility.
