Dave N. has prepared a list of some common mistakes related to SEO that one should avoid. Favorites:
1. Leaving the non-www version and the www live simultaneously.
2. Using ‘click here’ and ‘read more’ as default choices for internal links, rather than more descriptive phrases containing keywords where appropriate.
3. Syndicating content to higher authority sites which are likely to be indexed before your own site and thus become canonical.
4. Duplicated meta descriptions, which encourage Google to create their own snippets which can be nonsensical and harm clickthrough rates.
5. Forgetting that 25% of all searches have never been seen before and that search queries are typically much longer than single words.