Of eggs, baskets, and similar things
Some of your may have read about Incredimail (a public company listed on NASDAQ) having their Adsense contract terminated, and thus losing a big chunck of their revenue (IncrediMail Receives Notice from Google Regarding AdSense Program). This should teach everyone an old lesson: don’t put all your eggs (of even most of them) in the same basket, and use other sources of advertising on your sites. You know where the sign-up button is 