It's a fun discussion between me and some collegues, but maybe you have some ideas.
What would be the most worse backlinks you can do to a site?
In my mind are coming the following so far:

• Automatic created backlinks using free backlinking services

• Backlinks from porn websites

• Backlinks with DR less than 5

...any more suggestions?

The discussion started while talking about the risks people take while using cheap "seo experts",..
Or if someone maybe not pay them :P