As a database designer, I found that sometimes you can design something into the data structures that were difficult, if not impossible, to take advantage of in the UX.

For example, a fully cross-linked net structure for addresses, where a person could be linked to one or more addresses, and an address could be linked to one or more people. It worked really well until the question of how to handle address changes came up: Do you create a new address for the person with the address change, or do you change the address so that anyone linked to it has the change? Definitely a UX question.