I'll repeat this one until I turn blue. It is a terrible mistake to develop an application with an empty database, or one with just a little 'sample data' in it. Develop your database with the maximum amount of data in it that you can predict is going to be in the production database: Then, your failure to provide the correct indexes will become flamingly obvious. If you've made a design error, it will hit you smack in the eye. All right. I'll admit that SQL Data Generator is very close to my heart, but the reason is that this, or a tool like it that generates correctly formatted and convincing data, will become your best ally when you are developing.

Other crimes

Allowing direct table-access to applications (madness because of security issues)

Denormalisation (you'll live to regret it when the application expands)