I see a lot of different web sites, with a lot of different problems. From manual actions, to huge drops after a major algorithm update, to hacked sites, to sites that block Google by accident. The ones that always make me wonder are the ones that aren't hit hard one night but rather see a slow but consistent decline in organic traffic from Google.
I am sure you have seen them yourself and I see thread in the Google Webmaster Help forums on this topic all the time.
Here is one chart from a site I reviewed recently, showing only Google organic traffic over time:
It makes you wonder, what was the specific thing that hurt this site? It probably was not one algorithm, nor one specific issue. It may have been competitors getting better. It may be that Google trusts this site less and less over time. It may be the specific site has been removing links over time, which is impacting their rankings. Or it may be the site is less relevant to users today than it was before.
Have you seen a lot of cases like this? What would you recommend?
Forum discussion at Google Webmaster Help.