How end users dream up database designs (poorly)
End users tend to think of database design as a direct one-to-one mapping of their spreadsheet designs. So when you need to design a database for someone, they submit their existing Excel solution. You, the clever DBA that you are, begin mentally dividing out tables, normalizing, finding keys, etc.
Meanwhile, the user keeps talking about your database as if it were this one table, congruent with his Excel spreadsheet.
It's really our job to get the requirements from the user using his spreadsheet lingo, while knowing it's not how the database will be designed. I don't blame business oriented end users for not thinking in terms of relational database design. That's not their job. It's our job to convert their model into a relational model.