Recently I got burned by adding a persistent field to one of my lists. Persistent fields are site columns added by SharePoint and configured in a particular way. These fields can be added to a list, library or a content type. Once they have been added, they cannot be removed or edited using the SharePoint UI or SharePoint Designer. Unfortunately you cannot tell by looking at the list of site columns which ones are persistent and which are not.
The following blog posts list the persistent fields. If you do get caught by one of them and need to remove it, please send e-mail to email@example.com with your site URL, the name of the list or library that contains the field, and the name of the field. We can remove it using a Powershell script.
Analyst, SharePoint team
University of Saskatchewan