Don’t participate in link schemes designed to increase your site’s ranking or PageRank. In particular, avoid links to web spammers or “bad neighborhoods” on the web, as your own ranking may be affected adversely by those links. (A quote from Google Webmaster Guidelines.)

Often it is very obvious which link is good and which link is not good. If the links points to some bad site or a site that is doing something fishy, you are doomed if you link there. Also sites that use questionable techniques are not recommenced as online friends.

But sometimes the badness of a link can be nicely hidden. Here is an example:

The other day I found a comment on this post High PR Web 2.0 Properties for Link Building in SEO :

Looked to me like a relevant comment. I checked the URL and it looked fine too, so I went ahead and approved the comment.

Fast forward to today. I found this little tool on www.davidairey, to check good and bad link neighborhoods:
Text Link Checker Tool. I keyed in my domain and wham -  it spit out all my links and gives some data on it. To my surprise I learned that I was linking to a bad neighborhood:

I thought “that is strange”, I am not aware that I ever linked to that domain. “Was my blog hacked? Did I use an infected plugin?” Well, nothing that serious. I scrolled up and found this:

As the comments on this blog are nofollow anyhow and this particular page (fwisp.com) is only a bookmarking page, that can be used by anyone, there is no real danger in it. But I added bad neighborhood link checking to my routines for all my sites and all my customers sites.

If you hire an SEO company, remember to ask them to also check your links according to this.