Here are a few grave mistakes you can intentionally make: Showing different content to search engines than your visitors to game your rankings (that's cloaking, a black-hat tactic). Related to cloaking is when you hide text and links again for higher ranking purposes, for example, you color the text with the same background color and you hide the links behind images or other elements. Another black-hat scheme is the doorway pages, which aim to occupy high positions but then redirect the visitors to the content they would not expect to find when they run the search. It is a common scheme mainly for unethical advertising purposes.
Here are some intentional but not so grave mistakes: Creating bigger titles and descriptions than the average competitor has and stuffing keywords in them; Manipulating traffic by buying social signals (likes, tweets, shares, comments); Over-optimizing page content, schema markup, and image alt tags; Manipulating schema markup to show features you don't have, i.e., five-star ratings; creating thin content only for affiliate purposes; redirecting visitors to other properties or redirecting old domains with irrelevant content to your pages.