I will list a few here: Automatically generated content when you copy and paste content found on the internet without curating it, or you have a software that generates articles that you upload on your page. Another case is when you have an auto-translation widget on your site that translates content from your original, natural language to whatever the result in the target language.
Participating in link schemes when you buy or sell links. You could be exchanging links with other pages, building a private blog network (PBN), selling links from within your articles (contextual), or sitewide (sidebar, footer). The same stands when buying links from external web pages (everywhere an incentive is found).
When you create thin content; pages that have no value and no reason to rank, or you wish they would rank but don't invest time to put some quality on them.