A Google Webmaster Help thread has a rare case of Google apparently banning a web site completely by accident and then reinstating it after being notified about the mishap.
The story is about a site hosted on a Google Sites platform at sites.google.com/site/waldorfwatch. It is about a type of education methodology that is being embraced by some across the globe.
One day, the site which has been ranking well in Google for several years, noticed it was delisted from Google completely and he had no idea why. He wrote:
The sudden disappearance of my most important site from the most important search service, Google Search, seems inexplicable. And, obviously, it is damaging to my site. Please, someone, help!
At the same time, he never set up Google Webmaster Tools, nor a sitemap and never needed to.
The issue had nothing to do with him, it was a Google error. Google's John Mueller said in the thread:
It looks like this was an issue one our side. It should be resolved in the meantime, and from what I can tell, pages from the site are now appearing in search again, so you should be all set soon! In general, with issues like these, it's always possible to submit a reconsideration request to have someone on our side take a look.
I am not sure why Google had an issue with this site but it did and the poor guy suffered with little traffic because of the issue.
This is not unheard of but it doesn't happen every day.
Forum discussion at Google Webmaster Help.