1. Purchased Links
No matter how hard you work to make your link look "natural," it will reside on the same site as hundreds of unnatural links and will lose value as a result. Additionally, there is no guarantee that a purchased link will stay on the site, and monthly fees drain your funds and leads to bad ROI over time.
A URL that redirects to a different one will never accumulate links of its own, so this is the most unnatural link imaginable. Google can clearly see when a redirect was put in place and it would be extremely obvious to them that any links accumulated afterward were completely unnatural. This is a terrible way to “hide” manipulation.
3. Link Exchanges
People who exchange links just to get links are also involved in other manipulative techniques, so these links pass little value and may eventually bring penalties. A link from a site that only gets links from exchanges is useless, because the site leaves a very clear footprint when it does so.
4. Overzealous or Mediocre Guest Posting
Google already has quality detection algorithms (panda) and they are getting better. A low-quality guest post on a high-quality site is rare, so low-quality guest posts aren’t going to offer a great deal of value. You can spend two hours on an article and get it posted to a site where most pages have an authority around 50, or spend two hours writing 12 articles that will pick up a page authority of one or two. The impact just isn’t worth the effort in the second case.
6. Interlinked Domains
Sites hosted on the same account are often on the same server, or similar servers, so that it is obvious they are all owned by the same entity. Getting around this means paying much more for hosting. And, assuming you are able to build a network large enough to pass any benefit and that looks completely natural, the amount of work you will end up doing will be far more than for a single site. Why write a high-quality article for a peripheral site you own when you could write it for your money site, or for a link from a high-quality site with age and reputation?