A lot of site owners got into some trouble thanks to the Google Penguin update that went live in early 2012. The update went after sites there were guilty of web spam tactics like overuse of exact match anchor text, low-quality link building, link schemes and more. Getting nailed with a search engine penalty (either algorithmic or manual) can be a deadly blow to any website and there is no guaranteed road to recovery. I feel bad for the DIY SEO site owners that weren’t actively trying to game the search engines, but didn’t realize what they were doing was putting their site at risk.