Developer failings from my experience (SQL/BI developer):

1. Excessive use of cursors. Some developers seem to prefer to work with one row of data at a time then wonder why the application runs slowly.

2. Indexes - too many or not enough. In my last couple of contracts I've spent some time showing developers how to view Execution Plans, and use the Index Tuning Wizard and Database Engine Tuning Advisor. Some of them didn't know the tools existed!

3. Use of the "sa" login everywhere

4. Using inappropriate data types. eg. CHAR(255) for surname fields which are obviously variable length, or not allowing enough characters for a field based on the sample data that they created

5. Documentation. What's that? 😉