1. Keyword flooding

The Error:
Trying to optimize a home page for all possible keywords. Often you will see Title tags for example loaded with 12+ keywords, where a webmaster is attempting to squeeze in all his/her keywords on the home page. A classic example of a little know-how being a dangerous thing!

What generally happens is not one of the 12+ words ever reach a high ranking for the reason that individually they can never get the keyword density or repetitions needed in order to rank highly. This is especially the case for popular terms. I laugh when I see spammers hiding loads of keywords in long lists, knowing that rather than improving their ranking they just make it worse!
Less, can mean a lot more when it comes to SEO in this respect.

The Solution:
Focus your home page for a MAXIMUM of three of your top keywords. If you have a particularly competitive field then make that just one or two keywords.

Concentrate on just those keywords on your home page and of course in your title tags. Eg. The TOWER home page (root) concentrates on 3 keyword phrases where it does very well in German searches. Internet Marketing', Webpromotion', and Suchmaschinenoptimierung' (search engine optimization). A newbie at SEO would also have added Suchmaschinen eintrag', Suchmaschinenranking', Suchmaschinen platzierung' and possibly more keywords to the title tag, and would have tried to optimize the home page for all the terms rather than spreading them throughout the site as we have done.

Summary:
Focus on your top three keywords (hopefully researched properly) for your home page, keep them to a maximum of three, however if you are really in a niche market with little competition, it is ok to go for up to 4 or 5. Try and keep your title tag to less than 7 words and make sure your text copy uses the three terms at least 3 times each. Don't forget EVERY page is a potential entry page from search engines so there is no need to cram everything in on your home page.

2. Header area duplication

The Error:
It is human nature to be a bit lazy when developing a website. One of the most common, yet devastating for search engine traffic, mistakes is when a webmaster uses save as' to work on a new content page but forgets to change the non-visible header area of a page in Dreamweaver or whatever.

I think we've all seen these sites. A whole site has something like widgets-for-sale.com' in the title on EVERY page. The meta tags are identical on every page. Only the visible content is different. Rarely however do separate pages have exactly the same theme or content. Every page can be optimized for different keywords whether major or minor and can of course be an entry point to your site from a search engine. It is such a waste and almost makes me cry when I see great sites using mydomain.com for a title on every page.

The Solution:
When developing a site, stick to a pattern. We will normally do the content first but we always make sure the last thing we do before moving on to a new content page is to make sure we have not only the content optimized, but the area as well. You will not find an identical title tag on our whole website, or meta description for that matter. Never forget that each page is an entry page and optimize each to the best of your ability.

Summary:
Never repeat titles or meta descriptions in a website. Treat each page as if it were the most important and optimize it thoroughly. Don't be tempted to leave the head area without optimization.

3. Unnecessary Framesets

The Error:
It is now rare that we will see a framed website and believe that the use of frames in anyway enhances the site, or that it is a practical necessity for a webmaster. It isn't so much that framed sites generally rank lower, it is that few webmasters know how to correctly optimize them. This might give you an idea of the scale of the problem. http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=
%22browser+does+not+support+frames+%22+&btnG=Google+Search

The majority of those 697,000 websites require search engine optimization as to be honest, their current optimization stinks. Not many of those sites are going to rank in the top 10 of anywhere. Just to have in your noframe tag "...browser does not support frames" Is a great way to never get your website found on a search engine.

The Solution:
Treat the noframe tag content as if it was a text version of your home page and optimize it as you would a normal website. Very important also is to link to your framed pages from your noframe area. Also for your framed pages consider javascript that will call the frame set should it be found orphaned in a search engine. Normally framed pages without the frameset, mean no navigation and not displayed as was initially intended. The following code placed in all framed pages is one solution and works on the majority of browsers.

There are more complex / better solutions which really wouldn't fit in the space I have here. Try http://www.netmechanic.com/news/vol5/javascript_no7.htm for a more complete solution.
Also be aware that you can achieve what a frameset does through the use of CSS layer positioning, iframes and other methods. Only use frames if you really, really have to.

Summary:
If you must use frames, make sure you optimize them properly. Use the noframe tag properly and thoroughly link to framed pages. On your framed pages use javascript to prevent them being called without the frameset.

4. Splash / Flash sites

The Error:
We often see poorly ranked sites that visually contain a lot of text but the text itself is not of the font variety but graphic. Great eye candy, but forget a high ranking and search engine traffic if that is the only text on a page. We would say at least half our clients used to suffer from overdoing graphic text. The main webmaster culprits for this are (surprise, surprise) adult sites, and also those targeting young markets where it is believed lots of graphics and eye candy is what impresses and sells (handy shops, games console websites, games software sites etc.)

The Solution:
Integrate normal text where you can. You can make text and text links look great with a bit of css formatting know-how. You do not need graphic text to make text look attractive nowadays. At least do not make your pages all graphic text. Leave something for the search engine spiders to find and index. This also applies to Flash sites. Rarely does everything have to be a flash object. You can quite often have text surrounding a Flash object without any negative effects.

Summary:
Web pages that contain no normal text, or very little text, simply will not rank highly unless there is a VERY strong link campaign running. Mix graphics and objects with text. It is really this simple, No text = No ranking.

5. Keywords not researched

The Error:
Unfortunately too many webmasters do not really bother using any of several keyword research tools. There are about 4 or 5 of them. Most, like the overture keyword research tool, are free. Many webmasters don't think they need to use them as they know what their site is about and don't need to research the top keywords. This is a big mistake. Another big mistake is either optimizing for too niche or too obscure a search term, or going the other way and going for a very broad term with millions of competing pages on a new site with a only a handful of incoming links. Both are common errors and can result in all on page optimisation and off-page optimisation criteria, through requesting links with the wrong link text for example, to be a complete waste of time. You either get too little traffic as you optimized for terms that are rarely searched for, or you go for the terms with millions of competing pages but you simply do not have the experience or Pagerank to be able to compete.

The Solution:
The balance is normally achieved through two or three word phrases in competitive areas and yet don't have millions of competing pages. These are found best by cross referencing the several keyword research databases to be found on the overture.com and through a fair bit of lateral thinking.