1. Thinking too small

2. Not using EXPLAIN

3. Choosing the wrong data types

4. Using persistent connections in PHP

5. Using a heavy DB abstraction layer

6. Not understanding storage engines

7. Not understanding index layouts

8. Not understanding how the query cache works

9. Using stored procedures improperly

10. Operating on an indexed column with a function

11. Having missing or useless indexes

12. Not being a join-fu master

13. Not accounting for deep scans

14. Doing SELECT COUNT(*) without WHERE on an InnoDB table

15. Not profiling or benchmarking

16. Not using AUTO_INCREMENT

17. Not using ON DUPLICATE KEY UPDATEK