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.
Re: Many Things Which Can Effect On SEO work.
All of the above can really hurt SEO. Keyword stuffing and spamming are the most important things that can hurt people. SEO is a gradual process and will never be something that is achieved over night.