These are some basics when it comes to development on SQL Server.
Insufficient change control testing -Potentially the worst mistake to make, where developers will use a test case. Do a full TPC benchmark.
Unrealistic test environments - Provide an adequate db for the developers. One easy way is to export the production statistics into the test db
No change control procedure - In busy software companies, version control for schemas and sql code is paramount
Not enough testing instances- Thorough SQL developers often keep 4 environments, DEV for development, TEST for unit testing, QA for pre-production testing, and PROD for production. Any less and you risk missing errors
No performance tracking - With all the counters and alerts handling , there's no excuse for missing early warning signs, not to mention custom software packages
Poor security management - SQL Server offers a host of access control mechanisms. In dbs I audit, I still see obvious security holes
Non-standard external environments - This is a bad practice where every database uses different versions, inconsistent aliases and non-standard/varying file locations.