Hi !
Here are many things which can effect on SEO work.
Things that will certainly hurt our SEO work, specially bad effect on Page Rankings, and they are:
Keyword Stuffing,
Hidden Text,
Invisible Links,
Doorway Pages,
Spam Blogs,
Cloaking,
Duplicate content, frames, JavaSricpt, and Flash pages.
Other practices of off page SEO that hurt Web Page Rankings include
Forum Spam,
Email Spam,
Blog Spam,
Link Farms,
FFA websites,
Paid Links,
Inappropriate Links to your website, and links from unrelated websites.
Bookmarks