Let the Database Do Its Job
Ok, this pet peeve is a biggie. Over and over I have seen database schemas that simply defined the table structures and some stored procedures. Most modern database systems support advanced features for maintaining database integrity.
If you are going to go through the trouble to define a database schema, please finish the job. The database will help you do your job if you simply define Relationships, Constraints and Triggers. There are good tools for helping you do this. I really like ERwin or Visio for designing databases.