1) Targeting the wrong keywords
2) Ignoring the Title tag
3) A Flash website without a html alternative
4) JavaScript Menus
5) Lack of consistency and maintenance
6) Concentrating too much on meta tags
7) Using only Images for Headings
Ignoring URLs
9) Backlink spamming
10) Lack of keywords in the content
Backlink spamming means you are getting hundreds of backlinks on daily basis.