Google, being the indisputable leader in search engines, is placing a high importance on the quality and relevancy of its search results.

Most especially now that the company is public property. In order to keep the shareholders and users of its engine happy, the quality of the returned results are given extreme importance.

For this same reason, doing the wrong things in the Ad Sense and other forms of advertisements, whether intentionally or unintentionally, will result in a severe penalty, may get you banned and even have your account terminated. Here are some you must avoid:

Hidden texts. Filling your advertisement page with texts too small to read, has the same color as the background and using css for the sole purpose of loading them with rich keywords content and copy will earn you a penalty.

Page cloaking. There is a common practice of using browser to bot sniffers to serve the bots a different page other than the page your visitors will see. Loading a page with a bot that a human user will never see is a definite no-no. This is tracking them to click on something that you want but they may not want to go to.

Multiple submissions. Submitting multiple copeis of your domain and pages is another thing to stay away from. For example, trying to submit a site under two separate URL's is the same as inviting trouble and even termination.

Likewise this a reason to avoid auto submitters for those who are receiving submissions. Better check first if your domain is submitted already. If you see it there, then move on. No point contemplaing whether to try and submit there again.

Link farms. The search engines know that you cannot controll your links in, But you can certainly control what you link to, Link farming has always been a rotten apple in the eyes of search engines, especially Google. That  is reason enough to try and avoid them.
Having a link higher than 100 an a single page will classify you as a link farm so limit your outgoing links.

Page rank for sale. If you have been online for quite some time, you will notice that there are some sites selling their PR links or trading them wiht other sites. If you are doing this expect a ban anytime in the future. It is okey to sell ads of gain the link. But doing it on direct advertisement of your page rank is a way to get on all of the search engines' bad sides.

Doorways. This is similar to cloaking pages. The common practice of a page loaded with choice keyword ads aimed at redirecting visitors to another "user-friendly" page is a big issue among search engines. There are many seo firms offering this kind of services. Now that you know what they actually are, avoid them at all costs.

Multiple domains having the same content. In case you are not aware of it, Search engines look at domains IP's, registry dates and other data. Having multiple domains with the same exact content is not something you can hide from them. The same goes with content multiplied many times on separate pages, sub domains and forwarding multiple domains to the same content.

Many of the above techniques apply to most search enines and is not entirely for Google only. Simply put, you should only build your site for the human users and not for bots.