promoting or installing software directly onto a user's machine, for example, spyware, malware or trojans.
Changing a user's browser preference without their permission.
Changing the behaviour of elements on a screen means they move, for example, forcing a user to click Ads they didn't intend to.
Using private blog networks, site owners might have a network of sites passing links to a single site.
Negative SEO attempts to downgrade a competitor by pointing numerous spam links at their website.
Creating fake rich snippet markup.