A financial services company I did some work for had an application that created most of its important tables on the fly. The app generated DDL when it needed to store a set of analysis data and ended up with thousands of tables named ANALYSIS_DATA_20091015_1, ANALYSIS_DATA_20091015_2, etc. I've seen a lot of bad designs, but this may have been the worst.

In my opinion, most bad DB designs I've seen these days are caused by developers working without a DBA. Second to this, they're caused by an admin DBA who doesn't have data modeling skills.