Over use of Graphics- Many people tend to use graphics to make their websites more attractive. But that too should have a limit. It is not a good practice to fill you page with graphics. Remember that images takes a lot of bandwidth and another major thing is that the load time of you page increases significantly. This will be frustrating to your users. So use minimal graphics and hence make your site professional, clean and quick.

Use of Pop-Ups- Popups are a nuisance for majority of the users. So do avoid using it. And many users already use popup blockers, so giving important information as popup will be a problem because it will be filtered and the user might not even see your message.

Color Combination - Many people use brighter and darker colors like red. That doesn't look professional and might keep your user away. So use lighter shades in your site.

Scattered and Unorganised content - See, for a website to become a success, visitors should find it useful. The content should be organised in such a way that the users can find it easily. So it is advisable to use a site-map in the site.

Over use of ads - Ads also become a nuisance when then they appears in large numbers. So minimise the use of ads and place it in strategic locations so that you will get the exposure for the ad as well as prevent it from being a hindrance to the user.

These are common mistakes committed by Webmasters, so now you are aware of it, make sure that you don't commit any one of these. If you have come across any other mistakes webmasters make, let me know as comments.