Matt Cutts posted on Google+
This was in response to seoroundtable.com and other sites and forums reporting instances of several sites seeing their search rankings drop.I saw a recent post where several sites were asking about their search rankings. The short explanation is that it turns out that our classifier for parked domains was reading from a couple files which mistakenly were empty. As a result, we classified some sites as parked when they weren’t.
I apologize for this; it looks like the issue is fixed now, and we’ll look into how to prevent this from happening again.
It is apparent that this happened not because of any 'over-optimization' on part of the webmasters, but it was simply the Google Search classifier for parked domains goofing up.
Hope Google fixes this too, soon!
Details on Google+.