Top Ten Mistakes in Web Design

This is a fairly old list but still very applicable in my opinion:
Jakob Nielsen's - Top Ten Mistakes in Web Design

1. Bad Search

2. PDF Files for Online Reading

3. Not Changing the Color of Visited Links

4. Non-Scannable Text

5. Fixed Font Size

6. Page Titles With Low Search Engine Visibility

7. Anything That Looks Like an Advertisement

8. Violating Design Conventions

9. Opening New Browser Windows

10. Not Answering Users' Questions