Over Optimization is harmful for your site

Seo experts sometimes optimize a site to increase the rank in search engine result .They makes so many mistake in the direction to increase the rank.And one of the common mistake is over optimization of page.
there are many factors behind the over optimization. I want to explain some factors about over optimization.
Keyword Stuffing - Google imposed a penalty for those pages which have more keyword density . keyword density should be 4% to 7% is suitable within a page.keyword density about 10% or maximum is not good. Maximum keyword density is a part of over optimization.

Hidden Text - Hidden text are those text which have the same background color of page. These text are invisible from human eyes. So avoid the use of hidden text.