For no good reason other can collateral damage, we've been suffering from various filter/penalties for a few months now.
Where we used to rank between 1 and 20, we now rank 40-200. Usually, our english-language results are listed somewhere after the french-language results but before the turkish-language results. So, there's no doubt that this is a penalty, rather than any kind of natural ranking.
Recently though, sometime in the last few weeks, I've noticed something different that I've not seen mentioned anywhere else: in every case that I've looked at, Google are now ranking and listing the wrong page. So for Widget X, for example, we now rank 87th with the page for Widget Y. The Widget X page is nowhere to be seen. As I say, this is true for every one of our pages. Google are always choosing the wrong page as the most relevent (usually Widget X will be mentioned once as a related product).
From what I've seen this seems to be a bit of a general pattern at the moment. Looking at the first few pages of Google's results, you soon see many many examples of them choosing the worng page from a site. Looks to me like an Over-Optimisation-Filter run seriously amok.
<sarcasm>Strange, you'd have thought it would be really really easy for a dumb algo to spot pages that are simply too relevant to be relevant.</sarcasm>