Thinking that they are DBAs and data modelers/designers when they have no formal indoctrination of any kind in those areas.
Thinking that their project doesn't require a DBA because that stuff is all easy/trivial.
Failure to properly discern between work that should be done in the database, and work that should be done in the app.
Not validating backups, or not backing up.
Embedding raw SQL in their code.
