I spotted two isolated issues where a Googler confirmed a weird bug in Google's web search & quality classification system. TheJack from Google posted responses in two different Google Webmaster Help threads.
On one he said a site was having issues showing up in Google's search results due to Google having "misclassified the site as a parked domain." He added:
Looks we misclassified the site as a parked domain. Apologies that you were hit by this bug. We're working on getting this fixed. Should be cleared up in the next couple days.
This specific problem was only effecting this particular site. Outside of Google there's no way to tell unfortunately when this type of issue crops up. These are very rare occurrences so best thing for folks to do is to post here if they are seeing very fishy things happening with their ranking.
I then spotted an additional thread within the same time frame where TheJack, a Google employee, said a similar thing:
The drop in ranking is in no way related to anything you've done. This was simply a bug in one of our classifiers. Being fixed now so you should see things go back to normal over the next couple weeks.
This was a very specific issue. It was just this particular site, no cause for alarm.
He recommends, in some cases, if you are having issues with your rankings, it's best to post them at Google's forums.