Over-optimization may be defined in SEO as the excessive use of factors known as being usually appreciated by search engine algorithms. The purpose of over-optimization is to improve webpages and sites ranking.
Over-optimization is hard to define because of the thin frontier between good optimization and over-optimization.
From the visitor’s point of view over-optimization often starts when visible content seems artificial or poorly written.
From Google’s point of view, it is when content (visible or not) and popularity seem artificial and manipulative.
From SEO pros’ point of view it is when the website or page is sanctioned and climb down in SERPs.
Google detects and penalyzes over-optimization.
Dynamic advices from Rand Fishkin about over-optimization: