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The vast majority of sites are constructed by folks who have little knowledge about how some of the basic things they do or don’t do can spell death for those creations in Google.
 

 
These unpardonable sins are completely avoidable and need just attention and intent. I take the liberty to place 10 mistakes that Google considers as unpardonable sins.
 
1. 100% Flash based websites – Although Google has started indexing flash content (if created as per Google’s instructions), it is still a far cry and the site gets heavily penalized.
2. Duplicate Content – Google loves everything original. The intention is to present searchers with unique content. Google with throw you out if the content is a copy from somewhere or you use the same content for multiple sites with the idea of fooling Google.
3. Canonical Domain – Google treats www and non-www pages as different pages. All the 4 pages below are treated as separate pages by Google and can lead to the big issue of duplicate content. To avoid this, be consistent in the usage. A search on this in Google will yield you tons of guides.

• www.domain.com
• domain.com/
• www.domain.com/index.php
• domain.com/index.php

4. Linking to bad neighborhoods / link farms – It is very important to know the sites which link back to your website. Sites with poor reputation can damage your reputation in Google as well. So be careful.
5. Cloaking – Google hates stuff that is created differently for it’s bot and for the human eye. It considers that as deceit. This is treated as a black hat technique.
6. Session ids in indexable URLs – Having session ids in URL is a complete no-no if you want to get those pages indexed. Rewrite the URLs if needed.
7. Hidden Text – Another black hat technique which you should avoid under all circumstances. Display the same content to the Google bot as well as your visitors.
 
The above ’sins’ apply as much to other search engines as to Google. So avoiding these practices can yield you rich dividends across the board.