Excessive use of flash and Animation which require more time to load and require installing special addons or file to view.

Use of tables to structure the components instead of DIV. They miss to use flexibility which can be implemented with use of DIV.

Starting design of website without any preplanned strategy or process.

Giving final outcome to client without testing whether all the components work or not.

too much use of free scripting which may increase the ratio of errors and stop website by validating the W3c standard.

lack of compressing different images and using the uncompressed one which requires more time compare to compress one.

I want to include some errors-
1.A website's content is what drives traffic to it. How the content is structured is what will make it a success or a failure. Users do not read unless absolutely necessary but scan through information and pick out points of interest on a web page. Some designers just put a block of text on the web page and totally neglect headings, sub-headings, bullets, keywords, paragraphs, etc.
2.Navigation within a website should be seamless. Users should be able to find their way around easily. While there is no standard for navigation within a website, especially now as more new web development technologies emerge, it is imperative to understand that navigation must be intuitive and consistent.
3.Excessive creativity can be just that. Excessive! Some designers take it to another level when creating websites by creating different designs for every web page within a website. This is by no means confusing to the user. And utterly annoying. No matter how outstanding and attractive a website is, if the overall look and feel is not consistent, users cannot relate to it and feel less in control. Thus, leaving as soon as they arrived.

I would like to add JQuery in the list, I personally think Jquery should be avoid; as it takes lots of time to load a page mounted with JQuery....   

Color of your site really matters. So always try with cool colors which can help user to stay on site. You can use white or blue. These both are really good color and you can set your special offer with eye caching colors.

I would like to highlight 11 web design blunders that web developers and designers makes.

1. Where's The Search Box?
2. Poor Readability & Legibility.
3. Unorganized Content Layout.
4. Bad Navigation.
5. Inconsistent Interface Design.
6. Unfriendly Screen Resolution.
7. Complicated Registration Forms.
8. Unscrupulous Use Of Images/Animations.
9. Cluttered Pages, More Whitespace.
10. No Background Music, Please!
11. Test, Test And Test.

mistakes are always happen by the human. So web designer also do some mistakes on web design.  There are some basic mistakes in web design.  Mostly designer forgot to create table or div tag for putting content on web page.  As well as your mentioned information is always awesome and nice.

Here are some common mistakes that we could make in web design are given here:-
1)Not Changing the Color of Visited Links.
2)Providing Bad Search results.
3)Page Titles With Low Search Engine Visibility.
4)Anything That Looks Like an Advertisement.
5)Violating Design Conventions.
Thnx...

These are some common mistakes


1. Bad Search
Overly literal search engines reduce usability in that they're unable to handle typos, plurals, hyphens, and other variants of the query terms.
2.Don't leave your body text alone without any images.
3. Reduce usage of Flash, unless it's needed to explain something. I personally don't really like websites which has Flash intro. They are really annoying & wasting my time.
4.body text should have colorful background. It only makes people tired of reading your articles, because the body text color are blended with background imag

According to me Poor navigation, Slow loading sites, Picking random colors for your website, Big blocks of text, Sites designed entirely in Flash, Contact information that is hard to find, No clear goal for each page, A website design that isn't flexible are main mistakes...

There are plenty of mistakes web designers (especially new ones) make when designing websites. Using the correct image format for the different images on your site is vital. Using the wrong image file format can either make your image files larger than they need to be or affect the quality of the final image.