Some people suggest that with their adsense product, they encourage you to write pages that look like what people want, that turn up high on a query, something good enough that people don't immediately hit 'back' but that don't answer the question. If you answer the user's question, they are much less likely to click on your adsense ads.

Now, sometimes it kinda works okay; I know one guy who writes a fair bit of technical documentation; documentation that he promotes the hell out of, and that gives him fairly high pagerank. The problem is, programmers don't click ads. So what did the guy do? he threw in some pages about 'free MP3 downloads' with some free mp3s on there... of legal, free music that nobody wants, of course, but he makes orders of magnitude more money from the ads on that page, that page that ranks highly (because it's on a high pagerank site, and because it's got all the keywords) but that has no content that anyone actually wants.

I mean, personally, I don't have a problem with making money misdirecting people who search for "free MP3s" - but it does make google objectively worse.