There are stated to be few ‘black hat’ practices which are not preferred by search engines like Google. If used knowingly or unknowingly, such practices may only make Google to penalize the site heavily. Hence, it would be wise for every website owner and SEO practitioner to not use black hat techniques when optimizing sites and to avoid them, although it may appear to be easy and can offer quick results, unlike that of white hat techniques, which are slow in providing results, but, highly effective.
What is to be avoided when optimizing a site?
Excessive exchanging or buying of links: Are you planning to visit a link farm? Then simply drop the idea, since it would not be worth the effort or try. You need to understand that the valuable links which you are to get for your site would be the ones which come from those authority sites present within your niche.
Keyword stuffing: You should not overuse keywords within your pages as it is likely to affect your site readability. If keywords are still used by Google as ranking factor or not, is debatable.
Mobile app interstitials: For downloading your app, if your mobile visitors are presented with full screen advert, then Google is likely to consider you as mobile unfriendly.
Annoying ads: The pleasure of going through your content is sure to be destroyed with anything present that is overly intrusive, while also having site speed slowed down.
Hidden links and texts: Several ways are present by which rankings can be manipulated which may not be seen by the user. However, this is something that Google can find out and penalize you for it. White text is better avoided on white background. Also, you should not position text off-screen, hide link in single character such as full stop or comma or set font size to 0.
Duplicate content: In case, two identical content pieces are found by Google, be it on your site or that in the other which you may not be aware about, it is likely to have only one of the pages to be indexed. You are to be wary of scraper sites, which may have your content automatically stolen and get it republished as its very own.
The above aspects are to be avoided, if you wish to have your site to impress the search engines and to be top of the SERPs.