Weird excerpts in serps after Google ban has ended

Maybe I should have posted it to White Hat SEO, but:

As I am basically doing White and Grey Hat SEO, recently came across a "brick and mortar" client whose site was completely banned from Google's index (site: expression gave back nothing). As I could not get any info about the past of this site, we started to use AdWords to get traffic from Google by any means. It turned out that the site had been banned in AdWords too, although based on the existing external links and the site itself, I could not identify the reason for that and although after we had made an inquiry to AdWords, they lifted the ban (in AdWords, and then in the natural seps too), they have not specified the reason, only told that it happened due to spamming techniques in general.
Now, that the site is getting back for natural serps as well, with the site: operator I can see some weird excepts obviously coming from a general forum site: it shows as if certain forum threads would be on our domain, although the are and most likely they have never been on this domain. Link to cached version is not shown.

Do you have any idea about this? Is this due to any black hat technique, or just the previous site administrator has screwed up something?

Thanks!

Either it was hacked or there was another site on this domain before the current one
Check web archive

Thanks, web.archive.org was something I really forgot to check, as I basically suspected that something had to be happened with the external links to get that seriously banned by Google. You know the domain of my client has nothing to see with that forum site, which forum is still functioning and alive too, and I still cannot clearly see which black hat / hacking technique would have the side effect I could perceive...