SEO is about making your website search engine friendly. Below is the list of top mistakes people tend to make while optimizing a site.
1) Choosing wrong keywords for your website.
2) Long Lenth of title, description and Keywords.
3) Duplicate Content.
4) Nesting of tables.
5) Use of Frames
6) Use of Excessive flash.
7) Use of Java Script.
8) Making Unrelevent links.
9) Exceesive Use of keywords in content
10) Exceesive anchor text links.
11) Don’t Update Sitemap Regularlly.
12) Don’t adding fresh Content to your website.
13) Use of Black hat Technologies.
14) 301 Redirect to domain names and broken links.
15) W3c validation for your website.
16) Domain name not search engine friendlly
17) Don’t have Proper use of keywords in the alt image.
18) Cut the bigger images into small parts.
19) Lack of Clear navigation
20) Poorly designed websites.
Hope you will never make these mistakes in your website and enjoy top ten ranking for your website.

very useful tips, thanks for sharing the information.................