A few years ago I created a feature for a client's WordPress installation.
It was a directory of staff members. You enter their name, role, department and contact details.
In other sections of the site, you can assign a staff member as the point of contact for a specific product, support or general inquiry.
Assigning a contact stored a copy of the contact details instead of a reference to the source.
This was extremely foolish because if the directory was updated, the contact details wouldn't automatically update with it. You'd need to go in and choose the same contact for the details to update.