Here is mine:
1. Too many indexes on a table
Developers didn’t check their codes and an execution plan too. They just go and create the indexes (Duplicate indexes too).
2. Improper memory setting
Improper min, max memory setting especially for 64-bit servers.
3. Improperly configured transaction log
Whenever I got a new server, I have seen log backups are failing because, there is no current database backup.
BACKUP LOG cannot be performed because there is no current database backup.
4. Poor backup strategy
No SLA for the backups (data) from the customer/Manager.
5. Company didn’t believe the DBAs
The important (wealth) project was not running fine. The company suddenly called a consultant even they didn’t consider the DBA but, finally the problem solved by the DBA.
6. Rebuilding an index run first then shrinks the database
I have seen in most of the servers a rebuild jobs run first then shrink database runs next.
7. Linked server issue
Often I got called from teams the linked server was not working fine. It’s return timeout error.
8. Poorly written queries using a Non SARG-able
9. No Standard operating procedures