This question is bugging my mind already for a while, and all of my attempts to find such a case failed horribly so far. I am simply unable to dig up any traces of a court decision - whether in US, Europe, whatever - that would feature an Internet archive (archive.org, Wayback Machine, and so on) snapshot of a web page - or an element on a web page - being decisive in any legal proceeding, except for patents.
Are you aware of such a case?