1. Hidden keyword’s: Don’t try to hide a list of your keywords in a very tiny font in the footer or in the same color as your website’s background. Google will find it sooner or later and flag your website as spam.
2. Purchase Links: Incoming links are an important factor from the SEO point of view, so web sites that pay for incoming links or publish links that have been paid for by another site are flagged as spam and removed from search results entirely. Now the question is: How Google knows which links are paid and which are not ? Google doesn’t know that, but Google knows that the rapid growth of incoming links isn’t natural behavior of web sites.