There is an interesting and amusing thread over at SEW . A punter asks, on the surface, an innocent question as to why his mate’s site has dropped out of Google.
A bunch of the regulars offer some suggestions for possible problems, and then on the second page, GoogleGuy appears and really wades in, revealing the site is linking to some very bad evil affiliate spammers.
Interesting that GoogleGuy would take the time to do some research on the site. Interesting that SEW  allow such specifics to be discussed. Interesting that a good number of other SEO’s didn’t catch the real problem. And amusing that the punter gets his butt kicked from very high up in such a public manner. At least he had the good grace to admit he’s been a bad boy.
The lesson here people, is to be careful who you link to and who they link to in turn. Reciprocal linking is bad, you don’t know who else they have requested a link from. And do you have the time and skills to research those link properly. It took GoogleGuy to find the real problem and a bunch of professional SEO’s missed it.