Found this on the site recently, something to think about. (i think it's revealing in many ways)
You can read more about it here: http://www.useit.com/alertbox/9605.html

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. PDF Files for Online Reading

Users hate coming across a PDF file while browsing, because it breaks their flow.

3. Not Changing the Color of Visited Links

A good grasp of past navigation helps you understand your current location, since it's the culmination of your journey.

4. Non-Scannable Text

A wall of text is deadly for an interactive experience. Intimidating. Boring. Painful to read.

5. Fixed Font Size

CSS style sheets unfortunately give websites the power to disable a Web browser's "change font size" button and specify a fixed font size.

6. Page Titles With Low Search Engine Visibility

Search is the most important way users discover websites. Search is also one of the most important ways users find their way around individual websites. The humble page title is your main tool to attract new visitors from search listings and to help your existing users to locate the specific pages that they need.

7. Anything That Looks Like an Advertisement

Selective attention is very powerful, and Web users have learned to stop paying attention to any ads that get in the way of their goal-driven navigation.

8. Violating Design Conventions

Consistency is one of the most powerful usability principles: when things always behave the same, users don't have to worry about what will happen. Instead, they know what will happen based on earlier experience. Every time you release an apple over Sir Isaac Newton, it will drop on his head. That's good.

9. Opening New Browser Windows

Opening up new browser windows is like a vacuum cleaner sales person who starts a visit by emptying an ash tray on the customer's carpet. Don't pollute my screen with any more windows, thanks (particularly since current operating systems have miserable window management).

10. Not Answering Users' Questions

Users are highly goal-driven on the Web. They visit sites because there's something they want to accomplish -- maybe even buy your product. The ultimate failure of a website is to fail to provide the information users are looking for.

thanks.nice list.but there are other mistakes .i don't know if these are the most important

1- Attempting web design when you have ZERO! prior knowledge of the internet, how it works, dns, webservers, websites or anything remotely technical and then opening up your own "Web design" company that consists of installing wordpress and saying your a "Web designer"

Originally Posted by lynxus

1- Attempting web design when you have ZERO! prior knowledge of the internet, how it works, dns, webservers, websites or anything remotely technical and then opening up your own "Web design" company that consists of installing wordpress and saying your a "Web designer"

Many people call themselves web designers/developers without understanding what it means and how to do it.

I think it's because of the wide spread of technology and affordable software. Just because you have or know Dreamweaver doesn't mean you can.
(maybe it's our educational system)

I do agree with you they though it quite simply just like how to maintain a old carburetor car com pared to a efi engine car.

Thanks como, reading this makes me think about many site owners that should have read this before they built their sites.

I also agree that too many people call them self "designers/developers". I remember on myspace (back when myspace was alive) that teenagers would make pages and offer services when the only knowledge they have is Photoshop and some basic html.. What a shame :P

Originally Posted by mantasx

(back when myspace was alive)

I like how you put it.

I simply hate when people forget about the second..
And I am always paying attention to the 3rd

What about #10?
That's a big one I think. Many times you see designers/developers fail to communicate, when in many ways it's all about communication!

I don't find #5 appropriate for the list, and I don't see how communication should be on the list at all. I agree it's very important, but that has nothing to do with the actual design, besides specifics from clients; which is naturally always part of the process.

Lot of Flash and weird sounds are quite annoying also, I try to stay away from those sites as much as I can.

Originally Posted by webcreationuk

Lot of Flash and weird sounds are quite annoying also, I try to stay away from those sites as much as I can.

I think sounds period are a general turn-off when it comes to websites. Place like Youtube are of course an exception.

Originally Posted by como

7. Anything That Looks Like an Advertisement

I came across a couple of sites that had pieces of content presented in pictures much alike banners, so I subconcously didn't even try to read them.

Originally Posted by Madman340

I don't find #5 appropriate for the list, and I don't see how communication should be on the list at all. I agree it's very important, but that has nothing to do with the actual design, besides specifics from clients; which is naturally always part of the process.

Thank you for your comments. I can see how #5 may not be so important for some people, but when you have to deal and build sites that are user friendly and have to meet different standards, like targeting different age groups and people with disabilities. when your site has to work on every browser and for every eye, this come to play as important feature.

I would have to disagree with you on communication comment thou. Being in design field for some time now, I'm convinced that design in any form is no more or less than just communication. Every color, shape or form communicate.

Thank you.

Thanks for the very helpful information.. I dont know much about this.! And im glad for your help..

Thanks!

I like that #7 is here. Thank you for posting.

Thanks for sharing the very nice information. And i think the other below is also mistake in Web Design.

1. Do not overuse Flash: apart from increasing the load time of your website, excessive usage of Flash might also annoy the visitors. Use it only if you must offer features that are not supported by static pages.

2. Do not play music: on the early years of the Internet web developers always tried to successfully integrate music into websites. Guess what, they failed miserably. Do not use music, period.

3. If you MUST play an audio file let the user start it: some situations might require an audio file.

4. Do not clutter your website with badges: first of all, badges of networks and communities make a site look very unprofessional.

5. Do not use a homepage that just launches the “real” website: the smaller the number of steps required for the user to access your content, the better.

6. Make sure to include contact details: there is nothing worse than a website that has no contact details. This is not bad only for the visitors, but also for yourself. You might lose important feedback along the way.

7. Do not break the “Back” button: this is a very basic principle of usability. Do not break the “Back” button under any circumstance. Opening new browser windows will break it, for instance, and some Javascript links might also break them.

Originally Posted by ericpond

Thanks for sharing the very nice information. And i think the other below is also mistake in Web Design.

1. Do not overuse Flash: apart from increasing the load time of your website, excessive usage of Flash might also annoy the visitors. Use it only if you must offer features that are not supported by static pages.

2. Do not play music: on the early years of the Internet web developers always tried to successfully integrate music into websites. Guess what, they failed miserably. Do not use music, period.

3. If you MUST play an audio file let the user start it: some situations might require an audio file.

4. Do not clutter your website with badges: first of all, badges of networks and communities make a site look very unprofessional.

5. Do not use a homepage that just launches the “real” website: the smaller the number of steps required for the user to access your content, the better.

6. Make sure to include contact details: there is nothing worse than a website that has no contact details. This is not bad only for the visitors, but also for yourself. You might lose important feedback along the way.

7. Do not break the “Back” button: this is a very basic principle of usability. Do not break the “Back” button under any circumstance. Opening new browser windows will break it, for instance, and some Javascript links might also break them.

I like that. very good points. Some simple things that many people overlook, some even think it's cool to have your site look like myspace!!!!

and ericpond - don't underline text that isn't a link text