Yesterday, Digg was completely delisted from Google's search index. Some thought it was an issue with their robots.txt file, some thought it was a Google bug and some had no idea what to think.
What was it? Google accidentally penalized the whole site. Yes, it was an accident.
Matt Cutts, Google's head of search spam, said on Hacker News, "we were tackling a spammer and inadvertently took action on the root page of digg.com."
Google released an official statement as well:
We're sorry about the inconvenience this morning to people trying to search for Digg. In the process of removing a spammy submitted link on Digg.com, we inadvertently applied the webspam action to the whole site. We're correcting this, and the fix should be deployed shortly.
Shortly after, someone responded to Matt on Hacker News asking, "if this would happen to a less popular site, what chances does a site-owner have of getting attention to this problem, and getting it fixed?"
Forum discussion at Hacker News.