Can you Over Optimize your website? – Yes!
Keyword density on your web pages is key to search engine success. Where you place your keywords and how you implement them is equally as important. But you can do what they call keyword stuffing which has nothing to do with turkeys and Christmas.
To ensure the right success of all the pages on your website you should analyse how often you are using target keywords. Over use of keywords can compromise your search engine position.
Recommended keyword density is from 3% to 7%, anything above this can start to look like keyword stuffing this will not go unnoticed by search engine.
Use tools like http://webconfs.com/keyword-density-checker.php to analyse you site. Recent work by Community Connections has come up with surprising results – try it for yourself put your site in and some competitors.
If you require more in depth help with keyword usage and placement please see Community Connections.