DIRTY SITE NAVIGATION
When search bot (mainly talking about Google bot ) hits web site home page or any other page it follows the other links of web site on page ( if links are follow ) .Suppose a page (page 1) of your website is linked to home page and an other page (page 11) of web site is linked to another one(page 111) but no page (page1 ,page11 ,page111) is back linked to home page nor mutually back linked then search bot is confused and cannot crawl your web site successfully .It will prevent search bot crawling your web site frequently and it will give a very bad impression on search engine .
Solution
Every page have a back link to homepage and it is much better having A top level navigation on every page so that all pages remain connected with Top level menu .
VERY HEAVY WEB PAGES WITH GRAPHICAL CONTENT
seo has nothing to do with heavy images with transparency ,flash animations and flash videos .Search bot crawls just simple text present on your web page .Suppose we have worked hard to create a very good looking webpage with flash animations and heavy images .But we forgot to measure the size of page that exceeded 2mb .Search bot may leave your page without crawling in this condition .
UNPROFESSIONAL WEB DESIGN
It is a important point .If you are using a bad seo template for your blog there are many problems in loading page .Though Google is changing its seo rule for layout design but now a days css designs are favourite for search engines .There are many advantages of using css in layout .Ohhhhh we are talking about bad things here so i tells you what is not good in web design.
Using frames in design
Lack of css knowledge
A LOT OF NOT FOUND (404 ERROR ) AND URL REDIRECTIONS
Some time there is a need to transfer a blog from one plate form to other such as blogger to wordpress .Some innocent and novice users do not care about not found and redirections .And during transfer of blog they get a lot of these errors .In this condition Google may stop crawling your blog for a long time and search engine position may also down .
A VERY BAD WEB SERVER
Do you know about two words “Downtime” and “Offline” .Downtime is the time when your web site host is not working and your web site is being offline means your web site is not available for Internet users as well as search engine crawlers .Suppose your web site is offline at a moment and at the same moment search-bot visit your webpage,What will happen?
crawler will return without crawl.OK .Now crawler again comes and find the same condition .What will happen ?It may be possible that your web site may not be crawled for a long time .
A LOT OF LINK EXCHANGE
In starting it is good to build back links via link exchange method but link exchange beyond a limit may be dangerous .Besides linking to pirate web sites and other illegal web sites may be harmful.
