Outright spam, like forum and blog comments. Exploiting web sites, or abusing trust relationships (like updates to WordPress plugins or browser extensions) to i

Outright spam, like forum and blog comments. Exploiting web sites, or abusing trust relationships (like updates to WordPress plugins or browser extensions) to inject spam into legitimate pages, or to create entire networks of spam pages within unaware sites. Launching deliberately obvious spam campaigns to discredit competing sites. That's black-hat SEO -- where the behavior itself is probably illegal, even without the intent.