Optimizing for the wrong keywords:
The biggest sin anyone can make in a SEO campaign is to choose the wrong keywords to optimize your site with. If your Site is ranking high for a keyword that is not being searched for then even a very high ranking cannot bring you any traffic.
Spamming:
Search Engines are getting smarter by the day. Over a period of time Search Engines have evolved from “Ignoring” spam to a point where they now 'penalize' websites for using spam techniques.
Creating search engine roadblocks:
If you are developing a website, it helps to know of ways you can prevent creating search engine roadblocks. Your SEO efforts may not get results if your site structure is such that search engines find it difficult to index your site easily.
Having less content / Non-original content:
Search engines thrive on text content. They are mainly looking for text content on your website and reward sites which have lots of easily accessible, non-duplicate original text content.
Using session IDs:
Session ID is a long string of jumbled characters appearing in the URL of your website which changes on each visit. They are usually used to track a visitor ’s online shopping cart contents. When a search engine crawler visits your website, your server assigns it a session ID and the crawler indexes your content and associates it with that session ID.

SEM Experts