Guy and Eitan list several common mistakes done by DBAs in SQL Server.

Some relevant links:

• Architecting Database Dev and Test Environments: Best Practices and Anti-Patterns for SQL Server
• Ola Hallengren's Maintenance Solution
• HADR My Jobs - Open Source Solution by Madeira
• Columnstore Indexes: Planning and Implementation
• TSQL Script to Check for Weak Passwords
• Script to Check for Max DOP Configuration Best Practice