If you’re into tweaking your web pages to get ranked high in the search engines, and then rely upon that traffic for income, you might want to check out this article:

http://www.forbes.com/home/technology/2007/04/29/sanar-google-skyfacet-tech-cx_ag_0430googhell.html

The article points out that the penalty for spamming the search engines is banishment to a supplemental index. The problem is you may not know why you got banished!