A DigitalPoint Forum thread has a copy of a email sent to a webmaster for violating Google's webmaster guidelines. The email specifically shows the webmaster which guidelines they are breaking, in this case, hiding text.
Here is a copy of the email:
Dear site owner or webmaster of somewifi.com,
While we were indexing your webpages, we detected that some of your pages were using techniques that are outside our quality guidelines, which can be found here: http://www.google.com/support/webmasters/bin/answer.py?answer=35769&hl=en.
This appears to be because your site has been modified by a third party.
Typically, the offending party gains access to an insecure directory that has open permissions. Many times, they will upload files or modify existing ones, which then show up as spam in our index.
The following is some example hidden text we found at http://somewifi.com/:
songs Power Of Quest download songs Thomas Newman buy mp3 Tied and Tickled Trio new mp3 AFI top mp3 Alex Lifeson dowland ALO (Animal Liberation Orchestra) instrumental Dark oscillators mp3 songs Distance music download Euskefeurat music download F.J.Haydn download Fair to Midland
In order to preserve the quality of our search engine, pages from somewifi.com are scheduled to be removed temporarily from our search results for at least 30 days.
We would prefer to keep your pages in Google's index. If you wish to be reconsidered, please correct or remove all pages (may not be limited to the examples provided) that are outside our quality guidelines. One potential remedy is to contact your web host technical support for assistance. For more information about security for webmasters, see http://googlewebmastercentral.blogspot.com/2008/04/my-sites-been-hacked-now-what.html. When such changes have been made, please visit https://www.google.com/webmasters/tools/reconsideration?hl=en to learn more and submit your site for reconsideration.
Sincerely, Google Search Quality Team
Google specifically emailed this webmaster because they thought it was done by a third party, i.e. a hack. Google wrote in the email, "this appears to be because your site has been modified by a third party." Google then informed the webmaster that in order to protect the safety of the Google searcher, they have removed the infected website temporarily.
It is nice to see specific examples of this in real life, so I thought it would be nice to share with you all.
Forum discussion at DigitalPoint Forums.