if you overoptimize your site you won't see it in the first 1000 positions in google. You can overoptimize by having the same keyword in all anchor texts of incoming links.
Over Optimization? what do you mean by that... Maybe you are talking about offpage optimization (link building) ... Too much link acquisition may affect your rankings. Onpage optimization is okay as long as it has quality and good contents. Easy to navigate and other stuffs that promote good rankings to your site.
If you over optimized your site with your keywords you might penalized by search engine by keyword spamming. If you get a thousands of links in a short period of time they may think that it's not normal.
its not over optimized but wrong strategic of optimizing.. if you do much more or over as what really should it be.. is bad like in optimizing a site..
seo means helping search engines, so if you help them more than there is no harm. if you use black hat tactics, of making quick links, or keyword stuffing etc, you will face the problems