For Matt, who I'm sure knows them all by heart:

1. Not following basic typographic rules
2. Being too creative with navigation
3. Creating a cluttered navigation system
4. Making sure the site requires certain technology to work
5. Thinking that accessibility is only about blind people
6. Ignoring web standards
7. Not keeping search engines in mind from the start
8. Basing the site structure on your organisation structure
9. Using grey text on grey background
10. Skipping the feasibility study

...via elearningpost