The point about taking manual actions is an interesting issue because only in some rare cases such things might happen because Google is using an algorithm which normally does things in an automated style. Of course some hot bits of news are spread over the net from time to time concerning very huge organizations that have lost ranks of got into trouble and the Google team may have taken direct actions by manually reviewing their tactics.
A lot of discussions have been dedicated to the latest Google Panda and Penguin updates. When Panda became fully active, some websites' ranks were replaced with other domains, not big penalties were talked about as webmasters mainly discussed that they had experienced some very big fluctuations in ranks but Penguin has been actually hitting some websites that had problems.
For example, a lot of sites that were heavily promoted through paid links or some sites that were loaded with duplicate contents have been hot very hard. Lots of pages getting deindexed, and some even returning back to the old Sandbox phase, which means they are being treated like newly launched sites, still needing some weeks and months of time to get optimized again.