Google's Eric Kuan from the Google spam team posted a response to one webmaster who said a competitor is spamming them with bad links and asked Google to provide examples of some bad links.
Eric did so in the Google Webmaster Help thread by showing a few links.
I always enjoy sharing these public examples because it gives those who have never received an unnatural link warning an idea of what links to stay away from. In this case, the webmaster is persistent about those links not being acquired by himself but a competitor trying to sabotage him.