IMHO the most common mistake is over-optimization. This is a standard approach for the most part of newbies that comes to Search Engine Optimization after reading some articles or blog posts.

It's the human nature. When you learn something new and engaging, you often try to use this techniques everywhere even if they don't fit the case.

This mistake is not so easy to fix. It requires experience.

Think as SEO is the worst mistake we can do. We have to point the attention on relevant content that our visitors could find interesting and share on the web. Then we could look at metatag, links, titles and code optimization.

I think one of the worst SEO mistakes is not to update yourself, if you do not update yourself the damage a website is that you can't find new opportunities for visibility. SEO is an Orchestra, Not an Instrument...

IMHO:

1. Using the same META tag and title for all pages;
2. Stop optimization when you arrive the result;
3. Not update yourself. In the ICT area is very important update yourself, is a unwritten rule.

The biggest mistake is not creating a site that people would naturally like to talk about. No great content.

Then comes another mistake which is not building links on a regular basis. Don't do on-page optimization and say all is done and sit waiting for traffic.