The second most stupid thing is reconfigure your webserver so that it acts differently when the Googlebots comes along. This is called cloaking and Google cleary doesn't like it: Cloaking, sneaky Javascript redirects, and doorway pages
There are sites that provide to-pay-for content or that want you to register so that they can associate surfing behaviour with a pseudonym. It may e. g. be the web presence of a high-profile magazine with a small circulation and a high subscription price. Such content is highly valuable to a small group of people. Their content would cleary rank highly for some specific keywords on Google.
So what do these people do to remain relevant on Google?
They reconfigure their webserver to let the "user agent" Googlebot access their site without registration.
So here's my first SEO hack: become the Googlebot once in a while. Download the Firefox Extension "User Agent Switcher" and add a user agent Googlebot whose user agent field simply reads "Googlebot". Surf as the Googlebot and be surprised on some sites.
How to find those sites through Google: if you can't locate the preview snippet on the page itself and the cached version of the page is vastly different from the found page - you know this webmaster is trying to cloak Google.
