Over optimization may be a mistake that is done by a lot of publishes across the world. Some of these mistakes are done intentionally due to them being novices in the SEO industry. The fact is, try to make the content as natural as possible and focus only in the minor things such as Meta tags, keywords, etc. If it looks good for a reader, it would look good for the search engines as well.
One common mistake amongst publishers is keyword stuffing. This mistake is common amongst publishers that are new to SEO and thinks that the more keywords that the article covers, the more beneficial it would be. The truth is, keyword stuffing is not only a bore for the readers but also a bore for the search engines as well. They are complicated enough to know which content is real which one is not.
Duplicate contents are a big no-no. Not only will search engine degrade sites having duplicate content, webmasters will find it hard to optimize these sites as well as the search engines will prefer the site that created the particular content first. Use tools such as CopyScape to check for duplicate contents.