Those just starting to learn SEO (and possibly even some more experienced users) may become overaggressive with keyword use and risk being labeled a 'keyword stuffer'. Many SEO tips and SEO techniques center around keyword placement, but using the keywords too often and when not relevant is referred to as keyword stuffing. A rule of thumb is often said to be if a keyword or keywords are between to 15 and 20% of the content. That's a lot of keyword usage and does not go unnoticed. Many search engines will ban your site for such an offense.
The good news to those seeking SEO help is that instances of keyword stuff are typically pretty obvious (15 - 20% is hard not to notice) so it is rare that the accusation of keyword stuffing is unwarranted.